Visible Stars in Andromeda Galaxy

Visible Stars in Andromeda Galaxy

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So when I look at images taken of Andromeda, I see a lot of individual stars.

Image by André van der Hoeven

It seems to me that the actual individual stars that far away would be too small to resolve, even with a big scope like the Hubble. So, I have to assume that these points of light that I see are either a) stars in our own galaxy, or b) other distant galaxies, like we would see in the Hubble Deep Field:

So I decided to think a little about this… so I chose what I presume is big, bright star: R136a1. This one is about 30 times the size of the sun, and 8.7 million times as bright. We can barely see this thing with a telescope, and it's 163,000 light years away. Andromeda however, is 2.5 million LY away, over 15 times as far, which would make the light… what… 15 squared… 235 times as faint?

I have to imagine that the glow from the galaxy comes from the scattering and reflecting of all the collective starlight inside the galaxy off the dust and gas and whatnot contained therein, and that the actual individual stars would be kind of like individual droplets of water in a cloud. Almost kind of like when you can briefly see the form of a cloud when lightning flashes inside it at night (even though the light really isn't coming from the droplets)

Am I even close to correct in thinking this?


There is some confusion here about the word "resolve". In astronomy, to resolve an object means either to establish details of its structure and physical extent, rather than seeing it as a point source; or it means to separate a single entity into its component parts.

The former depends on the size of the object in question and how far away it is. The Hubble Space Telescope (HST) cannot resolve stars in this sense, not even the closest stars$^1$. All stars are imaged by HST as points of light, that are blurred to some extent because of the finite size of the telescope mirror and imperfections in the optics.

From that point of view, it does not matter how far away a star is, its image will look the same, other than of course the further a star is away, the weaker the total signal received will be.

I think your question refers to the second definition of resolution. Here the problem is to identify individual stars against a bright background. This can be done in Andromeda, because although stars are seen against a backdrop of billions of unresolved stars, astrophysics (as well as telescope optics) comes to the rescue.

There are relatively few very massive, evolved "supergiants" in a galaxy. But these objects are many orders of magnitude brighter than the stars around them. To "resolve" these stars, it is simply necessary to obtain images where the brightness of these individual stars is not smeared over too great an area so that they merge into the background light contributed by all the other stars.

This is actually not so difficult for a galaxy as close as Andromeda, and had been done using normal (but still large!) telescopes in the 1920s. The HST makes it much easier (and possible in more distant galaxies) because the light from an individual star is much less blurred by HST's optics and its position above our turbulent atmosphere. This enhances the contrast between individual bright stars and the bright background that they sit in.

$^1$ Actually, some of the closest supergiants like Mira and Betelgeuse can just about be made out as fuzzy blobs in the best HST images.

You are right that most of the dots of light that you see in the at image of Andromeda are stars in our galaxy that happen to be on the same alignment.

However we can resolve stars in the Andromeda galaxy. (Here I am understanding "resolve" means "to see as individuals and not just a haze")

Even without the Hubble telescope, individual stars had been identified in the Andromeda galaxy, and (since the brightness of some of these stars can be determined) the distance was calculated. This was also done by Hubble, but by the man, not the telescope. He found that the Andromeda galaxy was not part of the Milky Way, but much further away.

Hubble's photograph from 1923 can be compared to a modern skysurvey image.

The light in the image that you show is the combined light from billions of stars. They are too dim to be seen at the resolution of your image, but combine to make what looks like a shining cloud. Just like a real cloud is made of lots of dots of water that are too small to see.

There are background galaxies. Some can be seen on the high resolution view I link above. One surprising thing is how small the area of the sky in the Hubble Deep field image is. It would be only a few pixels across on the scale of the image you have of the Andromeda galaxy.

You could expect that many galaxies to be in every few pixels, but too dim to be seen in the image of Andromeda.

The question comes near the centennial of a famous debate about the nature of so-called spiral nebulae. In 1920, Shapley argued that that they were clouds within our own galaxy, and Curtis argued that they were distant galaxies in their own right.

Edwin Hubble's observations a few years later settled the issue. Using the 2.5 m reflector at Mt. Wilson to study Cepheid variable stars in M31, M33, and other nearby galaxies, he showed that they were well outside this galaxy.

The Wolf-Rayet star R136a1 is apparent magnitude 12.2, within reach of a skilled amateur with a 20 cm telescope. If it were at the same distance as M31, it would be only 6 magnitudes fainter, at magnitude 18.2. Automated 1 m telescopes routinely discover asteroids at magnitude 20 or fainter.

I have to imagine that the glow from the galaxy comes from the scattering and reflecting of all the collective starlight inside the galaxy off the dust and gas and whatnot contained therein, and that the actual individual stars would be kind of like individual droplets of water in a cloud.

This is not correct. The glow from from the galaxy is overwhelmingly light from individual stars traveling directly to us. This is demonstrated by the fact that galaxies lacking dust and gas, like most elliptical galaxies, have the same kind of "glow". (Examples: the compact blob of white light directly to the left of the center of Andromeda, about 1/4 of the image width away, is M32, a so-called "compact elliptical galaxy" in orbit about Andromeda. The more elongated, slightly more diffuse blob at 4 o'clock, about twice as far away from the center of Andromeda, is M110, a "dwarf elliptical galaxy" also orbiting the Andromeda galaxy. These are at roughly the same distance from us as Andromeda, but have essentially no gas or dust.)

What you are missing is the fact that the telescope optics (plus turbulence in the Earth's atmosphere for ground-based telescopes) blurs each individual star into a fuzzy disk with a finite size (each "disk" being bright in its center and fading away with increasing radius). These disks are large enough in angular size that -- if there are enough stars in a small enough area on the sky -- they overlap with each other, creating the appearance of a smooth glow.

You are correct that individual stars, even massive stars like R136a, are individually rather faint at the distance of the Andromeda galaxy; but there are hundreds of billions of stars in the Andromeda galaxy. Even in a small subsection of the galaxy, you are still seeing billions of stars. The combined, overlapping light traveling from those stars to the telescope is what we see.

Now, there are cases where you can see scattered/reflected light from dust, which has a lovely blue color (because dust scatters blue light more efficiently). But this is really faint, and generally only seen for nearby dust in our own galaxy. (Look for pictures of the Pleiades.) The faint blue regions in the outer parts of the Andromeda galaxy are not from this -- they are regions with lots of bright blue stars, due to there being lots of recent star formation in those parts of the galaxy. (Generally, only short-lived massive stars are hot enough to be visibly blue.)

In many galaxies, you can also see -- if you look at just the right wavelengths -- light emitted by individual gas ions/atoms/molecules in the interstellar medium. In the Andromeda picture you show, you can see faint pinkish blobs, mostly in the upper right of the galaxy; this is H-alpha emission from hydrogen atoms in ionized nebulae within the galaxy. This is somewhat exaggerated in the picture, because the person who took it deliberately combined the broad-band R, G, and B filters with separate exposures through a narrow-band filter centered on the H-alpha wavelength.

How can I see the Andromeda Galaxy?

Our galactic neighbour is visible all year from the UK, but clearest during the dark winter months.

Published: 08th December, 2020 at 12:05

At 2.5 million light-years from Earth, the Andromeda Galaxy is the most distant object visible with the naked eye. It’s the closest major galaxy to the Milky Way, and can only be seen if you have a really dark sky. However, the good news is that it’s visible all year round from the UK.

To find Andromeda, it’s easiest to start with the constellation Cassiopeia. For northern hemisphere stargazers, Cassiopeia is what’s known as a ‘circumpolar’ constellation, which means that it’s always visible above the horizon. Look towards the northeast and you’ll recognise Cassiopeia by the distinctive ‘W’ star pattern (or ‘asterism’) that its five brightest stars make.

Once you’ve found Cassiopeia, you can use the right-hand half of the ‘W’ as an arrow pointing towards Andromeda. The distance between Cassiopeia and Andromeda is about three times the height of the W. With the naked eye, Andromeda will be extremely faint. But if you have a pair of binoculars, look through them and you’ll see what looks like a cloud. That’s an entire galaxy.

While you’re in this part of the sky, you can also use the nearby ‘Great Square of Pegasus’ asterism to test the light population in your area. The Great Square is visible in the UK between August and December, and during October it can be seen all night. It has four bright stars arranged in an almost perfect square shape, and you’ll find it below and to the right of Cassiopeia’s ‘W’.

Once you find the Great Square, let your eyes adjust and then count the number of stars you can see inside it. If you see no stars, it means the light pollution in your area is poor. The average number of stars is 4, 9 stars is good and 21 is excellent. The most you’ll ever see with the naked eye, in the darkest skies, is 35.

Looking for stargazing tips? Check out our complete astronomy for beginners UK guide.

Hubble: Andromeda Is Big, Massive, And Full Of The Stars Our Milky Way Is Missing

The Milky Way is our cosmic home, containing hundreds of billions of stars across 100,000 light-years.

But 2.5 million light years away, our big sister, Andromeda, outclasses us in every way.

It’s double our diameter, with around a trillion stars.

It’s the Local Group’s biggest, most massive and luminous galaxy.

When we look at the stars within Andromeda, with space telescopes like Hubble, the biggest differences emerge.

Over 117 million stars in the disk were measured by PHAT: the Panchromatic Hubble Andromeda Treasury.

The stars near the central bulge are far richer in heavy elements than our Sun.

New, blue stars shine in a slew of open clusters.

The low-density, outer halo contains stars just as ancient as the Milky Way’s oldest: 13+ billion years of age.

Andromeda has stellar streams populating that halo, with a third of those stars just 6–8 billion years old.

2 billion years old in both M32 and Andromeda. The stars found in the halo and stellar streams of M31 point to another such merger even earlier: 6–8 billion years ago.(AMANDA SMITH, INSTITUTE OF ASTRONOMY, UNIVERSITY OF CAMBRIDGE)

This means a major act of galactic cannibalism recently occurred.

Ultraviolet images showcase the newest stars, tracing out spiral arms and peaking in the center.

Infrared imaging pinpoints the galactic fuel that will birth future generations of stars.

Thousands of background galaxies, seen through Andromeda’s halo, showcase our chaotic, evolving Universe.

Mostly Mute Monday tells the cosmic story of an astronomical object or phenomenon in images, visuals, and no more than 200 words. Talk less smile more.

Where is the Andromeda Galaxy?

Unsurprisingly and unironically the Andromeda Galaxy is located within the constellation of Andromeda. This is best viewed in fall in the Northern Hemisphere where it is usually viewable during all dark conditions from dusk until dawn, in perfect conditions.

In the middle of fall, late September until early October, the Andromeda galaxy will rise in the eastern sky and will stay overhead around midnight and will fade in the west as dawn breaks. Much like our moon does as our Earth turns.

During the winter months, Andromeda is viewable overhead and you can use planispheres or certain astronomy software for assistance in spotting it in the night sky overhead. If you are looking up and you cannot see the galaxy, but you know you are looking at the right time, you can star-hop to find it.

One of the easiest recommended ways to do so is to the constellation of Cassiopeia, this constellation is easy to use for this, as it is easy to find, shaped like the letter ‘M’, it is northward on the sky’s dome.

Or if you know where the Big Dipper is, then you can use that the Big Dipper and Cassiopeia move around Polaris (the North Star) much like the hands of a clock, always opposite each other.

Visible Stars in Andromeda Galaxy - Astronomy

Surprising stars in the Andromeda Galaxy
Posted: 13 January 2012

Greater insights by the Hubble Space Telescope into the heart of the Andromeda Galaxy, the closest large galaxy to our Milky Way, are revealing a mixed environment of unusually blue stars and a ring of red stars around its giant 100 million solar mass black hole.

The Andromeda Galaxy with the new Hubble image of its core, featuring the elliptical ring of red stars and the compact cluster of blue stars. M31 image: WIYN/KPNO Hubble image: NASA/ESA/T Lauer (NOAO).

The central bulge of the Andromeda Galaxy (M31) is home to many old, red stars. So what are a population of very blue stars doing within the bulge? As part of the Panchromatic Hubble Andromeda Treasury survey to map different types of star across M31, a team led by Julianne Dalcanton of the University of Washington in Seattle discovered about 8,000 blue stars that have a strong ultraviolet component. Typically blue stars are hot young stars, but given that these stars are in the bulge where the older residents of galaxies lie, they must be elderly stars that have had their outer layers removed, exposing their hot interiors. The fact that they are dimmer and posses a range of surface temperatures different to the young blue stars in M31’s star-forming regions backs up this hypothesis.

The results, however, which were presented at the American Astronomical Society meeting in Austin, Texas, have created something of a paradox. The distribution of the anomalous blue stars across 2,600 light years within the centre of M31 matches the distribution of X-ray binary stars within the bulge as seen by NASA’s Chandra X-ray Observatory. X-ray binaries involve one star stripping material from a smaller companion star, causing the gas to flow into a hot accretion disc around the stellar thief. The removal of material from the smaller star exposes the hotter, bluer layers underneath. However, if X-ray binaries were to blame, we would see similar stars with a blue/ultraviolet excess of light in other large galaxies where binary systems reside, and this just isn’t the case. This has left astronomers searching for other explanations.

One idea is that the stars contain more helium in their cores, making the core hotter (higher temperatures are required for the nuclear fusion of helium rather than hydrogen) and this would cook the atmosphere from the inside out, says team member Phil Rosenfield, a graduate student at the University of Washington. However, their favourite explanation, Rosenfield says, is that these stars contain more heavy elements or ‘metals’ – elements with atomic masses greater than hydrogen and helium. The extra heavy elements help drive stronger stellar winds that can lift off an evolved red giant star’s outer layers.

Blue stars caught lurking in the central bulge of the Andromeda Galaxy. Image: NASA/ESA/B Williams and J Dalcanton (University of Washington, Seattle).

A red giant is a phase late on in a star’s life where it begins to deplete its store of hydrogen and swell to a size many times what it was, its surface growing cooler and redder as it becomes bloated. As the red giant’s energy source alternates between helium and hydrogen burning, the star pulsates, pushing heavy elements up to the surface of the star. “Think of sea foam being pushed up a shore by waves,” Rosenfield tells Astronomy Now . “Away from the surface, the heavy elements can cool and form dust.”

The dust then absorbs the star’s radiation, and the transferral of momentum from the stellar photons to the dust causes the dust to move away from the star on the stellar wind, dragging large amounts of gas with it. The greater abundance of heavy elements in the stars seen in M31 mean that more of the outer layers are lifted off than on a typical red giant, allowing more of their hot interior to be exposed. Exactly why these stars are so metal-rich is not explained, but the team are now working on computer simulations to attempt to determine which explanation is the correct one.

Hubble has meanwhile delved even deeper into the centre of M31 to take the sharpest visible light image ever taken of the region immediately around its supermassive black hole. Although the black hole’s event horizon – its outer boundary inside which nothing can escape – is too small to be resolved even by Hubble, the image does show a cluster of blue stars swarming around it. Unlike the evolved blue stars discovered by the Panchromatic Hubble Andromeda Treasury survey, these stars are definitely very young, perhaps no more than 200 million years old and formed in situ around the black hole. Our Milky Way also harbours clusters of hot, blue, newly formed stars around its central black hole and the new findings suggest that such clusters may be common around black holes inside spiral galaxies.

Encircling this blue cluster is an elliptical ring of ancient red stars, initially discovered by the Hubble Space Telescope in 1992. Back then it was thought that M31 had some bizarre double nucleus, but this was an optical illusion created by the stars moving slower at the farthest point in the ring from the black hole, giving the impression they were part of a separate structure rather than part of a single ring. The new image is the work of Tod Lauer of the National Optical Astronomy Observatory in Arizona, who combined images taken in blue and ultraviolet light taken by by Hubble in 2005 and 2006.

Andromeda Galaxy Visible to Eagle-Eye Skywatchers

Soonafter sunset, as the sky gets dark, look toward the south and you?llimmediately notice the brilliant planet Jupiter, but there's another night skytarget that also promises a great experience: the amazing Andromeda Galaxy.

Jupiteris a great starting point to find the AndromedaGalaxy. After you find the bright planet, look high above it ? almostdirectly overhead - to find four bright stars. These are the Great Square ofPegasus, the Winged Horse, an unmistakable star pattern, even though it isslightly battered out of true square shape.?

Nonetheless,Pegasus is a striking figure and once you see it you won?t forget it. [Gallery:Photos of the Andromeda Galaxy]

Interestingly,the star in the upper left corner of the Square ? Alpheratz ? actually belongsofficially to the constellation Andromeda. Andromeda, according to legend,should be chained to a rock.? Instead she seems to have become chained to thehorse: a double strand of stars ? one strand bright, the other dim ? connectedto the upper left corner of the square.?

Aboutmidway and above these star strands is the Andromeda Galaxy, one of the mostamazing and fascinating of sky objects. It can be seen faintly with the unaidedeye, but binoculars and small telescopes promise better viewing.

Thissky map shows whereto look to see the Andromeda Galaxy over the next week, though clear weather isvital to spot it.

'LittleCloud' actually a neighbor galaxy

Inthe year 905 A.D., the Persian astronomer Al Sufi drew attention amidst thestars of the Andromeda constellation to a "Little Cloud," which appearedon star charts long before the telescope was invented in 1609.?

Ifthe sky is clear and moonless you can indeed see an elongated hazy patch withyour unaided eye about as long as the width of the full moon and half as wide.

Throughbinocularsand telescopes it remains an elongated patch which gradually brightens inthe center to a star-like nucleus. It was listed as object number 31 in CharlesMessier?s 18th century catalogue of galaxies,nebulas and star clusters,which is why it is known as Messier 31 or M31. We know it better as theAndromeda Galaxy.?

Thebest way to positively find theAndromeda Galaxy is to focus your eyes or binoculars on Alpheratz. Runstraight across to the left and get the star Mirach in your field of view.?

Thenrun slowly upwards to a fairly bright star above Mirach and continue to run upin roughly the same direction and same distance. You?ll immediately take noteof a little patch of faint light. ?

Congratulations!?You?ve found Messier 31.?

Pleaseforgive this patch of light for being so faint and tired looking. It's amazingto realize that, as you see it tonight, this light has been traveling some 2.5million years to reach you (give or take a few hundred thousand years),traveling all that time at the tremendous velocity of about 671 million mph ?the speed of light.

Thelight from that "Little Cloud" is actually the total accumulation oflight from over 400 billion stars.

Asyou look at the Andromeda Galaxy tonight you'll be doing something that no oneelse in the world except a stargazer can do: You will actually be looking backinto the distant past. Thelight you are seeing is around 25,000 centuries old and began its journeyaround the time of the dawn of human consciousness.?

Whenit began its nearly 15 quintillion(that's 15, followed by 18 zeros!)mile journey earthward, mastodons and saber-toothed tigers roamed over much ofpre-ice-age North America and prehistoric man struggled for existence in whatis now the Olduvai Gorge of East Africa.

TheAndromeda Galaxy is the most distant object that can be seen with the unaidedeye.

M31has been estimated to be nearly 200,000 light-years in diameter, about 1 1/2 timesas wide as our own MilkyWay galaxy. Its bright nucleus is the hazy patch that is visible to theunaided eye.?

Likeour own galaxy, M31 has several attendant satellite galaxies. Two of these: M32and M110 can be picked out with low magnification in a small-to-medium sizedtelescope, in the same field of view as M31.?

Fora very long time, M31 was popularly referred to as the Andromeda"Nebula."

Althoughbig reflecting telescopes, such as Lord Rosse's 72-inch telescope at BirrCastle in Ireland, were in operation during the mid-19th century, it was notseen clearly until 1924,when astronomers Edwin Hubble and Milton Humason used the 100-inch reflectingTelescope at Mount Wilson Observatory to become the first persons to resolveM31 into individual stars.

Yetthere were those who many decades earlier suspected that M31 was much more thanjust a luminous cloud.? Read this prophetic comment out of W.H. Smyth's "ACycle of Celestial Objects" written back in 1844:

"SirJohn Herschel ? concludes that it is a flat ring, of enormous dimensions, seenvery obliquely. It consists probably, of myriads of solar systems at a mostastounding distance from ours, and affords a distinct lesson that we must notlimit the bounds of the universe by the limits of our senses."?

Thisgalactic neighbor of ours, the Andromeda Galaxy, is the nearest of all thespiral galaxies to us and one of the largest known.

Joe Raoserves as an instructor and guest lecturer at New York's Hayden Planetarium. Hewrites about astronomy for The New York Times and other publications, and he isalso an on-camera meteorologist for News 12 Westchester, New York.

Visible Stars in Andromeda Galaxy - Astronomy

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Star Clusters in the Andromeda Galaxy

Fast Facts

2.5 million light-years (0.8 megaparsecs)

Data of M31 were obtained from the HST PHAT Treasury Proposals: P.I. J. Dalcanton (University of Washington) et al. 12055, 12056, 12057, 12058, 12059, 12076, 12070, 12071, 12072, 12073, 12074, 12075, 12114, 12105, 12106, 12107, 12108, 12109, 12111, 12112, 12113, 12114, and 12115.

The science team comprises: D. Weisz and L.C. Johnson (University of Washington), D. Foreman-Mackey (New York University), A. Dolphin (Raytheon Company), L. Beerman, B. Williams, and J. Dalcanton (University of Washington), H.-W. Rix (Max Planck Institute for Astronomy, Heidelberg), D. Hogg (New York University/Max Planck Institute for Astronomy, Heidelberg), M. Fouesneau (Max Planck Institute for Astronomy, Heidelberg), B. Johnson (Harvard-Smithsonian Center for Astrophysics), E. Bell (University of Michigan), M. Boyer (STScI), D. Gouliermis (Max Planck Institute for Astronomy/University of Heidelberg), P. Guhathakurta (University of California, Santa Cruz), J. Kalirai (STScI), A. Lewis (University of Washington), A. Seth (University of Utah), and E. Skillman (University of Minnesota).

Bottom images: F336W (U), F475W (g), F814W (I), and F160W (H) Top Image: F475W (g) and F814W (I)

NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler

These images are composites of separate exposures acquired by the ACS and WFC3 instruments on the Hubble Space Telescope. Several filters were used to sample broad wavelength ranges. The color results from assigning different hues (colors) to each monochromatic (grayscale) image associated with an individual filter. In this case, the assigned colors are:

Blue: WFC3/UVIS F336W (U)
Green: ACS/WFC F475W (g)
Yellow: ACS/WFC F814W (I)
Red: WFC3/IR F160W (H)

[Top] – This is a Hubble Space Telescope mosaic of 414 photographs of the nearest major galaxy to our Milky Way galaxy, the Andromeda galaxy (M31). The vast panorama was assembled from nearly 8,000 separate exposures taken in near-ultraviolet, visible, and near-infrared light. Embedded within this view are 2,753 star clusters. The view is 61,600 light-years across and contains images of 117 million stars in the galaxy's disk.

[Bottom-Left] – An enlargement of the boxed field in the top image reveals myriad stars and numerous open star clusters as bright blue knots. Hubble's bird's-eye view of M31 allowed astronomers to conduct a larger-than-ever sampling of star clusters that are all at the same distance from Earth, 2.5 million light-years. The view is 4,400 light-years across.

[Bottom-Right] – This is a view of six bright blue clusters extracted from the field. Hubble astronomers discovered that, for whatever reason, nature apparently cooks up stars with a consistent distribution from massive stars to small stars (blue supergiants to red dwarfs). This remains a constant across the galaxy, despite the fact that the clusters vary in mass by a factor of 10 and range in age from 4 million to 24 million years old. Each cluster square is 150 light-years across.

Charting the Andromeda Galaxy

By: Monica Young January 6, 2015 0

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The Hubble Space Telescope has turned its ultraviolet, visible-light, and near-infrared eyes to the queen of galaxies, M31, capturing the biggest and sharpest image yet of our neighbor.

At the winter American Astronomical Society meeting this week in Seattle, a poster of the Andromeda Galaxy welcomes astronomers to the biggest astronomy conference of the year. The poster is something like 10 feet tall and 25 feet wide — and that doesn’t even do the image justice.

This composite image of M31, the Andromeda Galaxy, is the largest ever assembled from Hubble Space Telescope observations. At full resolution, you can see individual stars, even though the galaxy is 2.5 million light-years away. Explore the high-res version with the zoom tool made available by the Hubble team.
NASA / ESA / J. Dalcanton / B.F. Williams / L.C. Johnson / PHAT / R. Gendler

The Hubble Space Telescope high-res image above captures a slice of Andromeda spanning 48,000 light-years, from bulge to outskirts. Its 1.5 billion pixels would need 600 HD television screens to display to full effect.

Hubble began studying Andromeda in December 2011 as part of the Panchromatic Hubble Andromeda Treasury (PHAT) project led by Julianne Dalcanton (University of Washington). The imaging project finished in November 2013, and the team released the result on January 5th at the meeting. The final image includes 12,834 shots from more than 400 pointings taken through ultraviolet, optical, and near-infrared filters. (The photo above shows only the visible-light view through the blue and red filters, a mosaic of roughly 3,700 optical images).

The team enlisted the help of well-known astrophotographer Robert Gendler, who stitched the images together to create the seamless mosaic. The stitching is so careful that the mosaic is aligned at the level of individual stars — roughly 117 million of them — or to better than one-tenth of an arcsecond. That’s not too shabby for charting a galaxy 2.5 million light-years away.

The result is a detailed look at our neighbor like we’ve never seen before, one best explored via the zoom tool on the European Space Agency’s Hubble site. (It takes a while to load, but it’s worth it.) Explore (and zoom, and zoom some more!) and you’ll see those 117 million stars, along with a couple thousand star clusters and star-forming regions as well as dark, twisted silhouettes traced by complex dust structures.

The Science Behind the Pretty Picture

This wide-field view shows the Andromeda Galaxy along with its companion NGC 205 (upper right). The irregularly edged swatch is the extent of the PHAT survey, which over 39 months took thousands of high-resolution images of Andromeda using the Hubble Space Telescope. The clean-edged rectangle is the image shown above.
M31 PHAT Mosaic Credit: NASA / ESA / J. Dalcanton (University of Washington) / B. F. Williams (University of Washington / L. C. Johnson (University of Washington / PHAT team / R. Gendler Credit for ground-based background Image of M31: © 2008 R. Gendler, Used with Permission

For Dalcanton, it’s the last item in that list — the twisted columns of obscuring gas and dust — that’s most interesting. Dalcanton has already used the image to map dust across the Andromeda Galaxy.

The team first divided the image into boxes 5 arcseconds (65 light-years) wide, each one containing foreground stars, background stars, and dust. As background starlight passes through intervening dust, it reddens just as a sunset reddens when passing through dust or smog. So for each box, Dalcanton’s team modeled the stars’ range of brightnesses and colors, and for each box they included two populations in their model: one reddened and one unreddened.

The result: a 3D dust map of the galaxy, one that has more than four times better resolution than previous dust-mapping methods. The team had to "fuzzify" the new dust map in order to compare it against other methods, but so far it’s in excellent agreement with previous charts in terms of the dusty structures’ shapes.

But surprisingly, the team found that other widely used dust maps actually predict twice as much dust as is really there. Dalcanton suggests a calibration issue with the other model as the most likely culprit. If that’s the case, nearby galaxies may have much less dust than previously thought.

Charting dust and its mysteries is essential to understanding starbirth, as dust helps to cool interstellar gas, and stars form from cool gas. This study is only the first from PHAT to aim for that ultimate charting goal. Forthcoming studies will study star formation as a function of position in the galaxy, investigate the galaxy’s star-formation history, and much more: “This is meant to be a legacy data set, to be used for decades,” Dalcanton says.

The Mystery Ring

Another surprise from the PHAT mapping is in the Andromeda Galaxy’s structure. Observations such as those in ultraviolet from NASA’s GALEX spacecraft and in infrared from the Spitzer Space Telescope reveal where stars are currently forming in the Andromeda Galaxy. As expected, star-forming regions riddled with young, massive stars trace out M31’s iconic spiral arms. The tightly wound arms — perhaps in some cases even genuine rings, like those created in a stone-disturbed pond — are likely a transient thing computer simulations show that such arms should move and evolve over time.

GALEX and Spitzer images show the lay of the stellar land “now” (well, when light left the galaxy 2.5 million years ago). But because the color and luminosity of stellar populations reveal the stars’ ages, and because these properties change as you look at different parts of the galaxy, the PHAT images actually enable astronomers to look back in time and determine M31’s star-forming history in various locations.

What the team found is that the arms aren’t all as transitory as expected: a ring present today was also forming stars between 500 and 630 million years ago, a time scale much longer than astronomers predicted for these structures to survive. The inner and outer rings vary as expected, but not this one.

“This was really a surprise,” Dalcanton said in a press conference. In terms of stellar content, the density of stars in this ring is about 40% higher than in other regions in Andromeda, and it contains both old and young stars — it’s not just the young stars tracing it out, as is common with spiral structure. “So it’s this long-lived dynamical thing that’s just kind of sitting there, for reasons we don’t understand.”

Learn more about the team’s results on the PHAT project website.

Science Editor Camille M. Carlisle contributed to the reporting and writing of this news blog.

Hubble Observes Rare Blue Stars in Andromeda’s Core

The image at left shows the nearby, majestic Andromeda galaxy. The rectangular box marks the region probed by NASA’s Hubble Space Telescope (a blend of visible and ultraviolet light). The photo (top right) is 7,900 light-years across and reveals the galaxy's crowded central region. The bright area near the center of the image is a grouping of stars nestled around the galaxy's black hole. The blue dots sprinkled throughout the image are ultra-blue stars whose population increases around the crowded hub. The square box shows a close-up view of an area around the core. The detailed image, shown at bottom right, reveals a richer population of blue stars huddled around the core.

NASA’s Hubble Space Telescope made a rare discovery when looking deep into the neighboring Andromeda galaxy. Spotted was a population of rare blue stars in a much broader area than ever seen before. Astronomers used Hubble’s Wide Field Camera 3 to find roughly 8,000 of these blue stars within 2,600 light-years of the core.

Blue is typically an indicator of hot, young stars. In this case, however, the stellar oddities are aging, sun-like stars that have prematurely cast off their outer layers of material, exposing their extremely blue-hot cores.

Astronomers were surprised when they spotted these stars because physical models show that only an unusual type of old star can be as hot and as bright in ultraviolet light.

While Hubble has spied these ultra-blue stars before in Andromeda, the new observation covers a much broader area, revealing that these stellar misfits are scattered throughout the galaxy’s bustling center. Astronomers used Hubble’s Wide Field Camera 3 to find roughly 8,000 of the ultra-blue stars in a stellar census made in ultraviolet light, which traces the glow of the hottest stars. The study is part of the multi-year Panchromatic Hubble Andromeda Treasury survey to map stellar populations across the galaxy.

“We were not looking for these stars. They stood out because they were bright in ultraviolet light and very different from the stars we expected to see,” said Julianne Dalcanton of the University of Washington in Seattle, leader of the Hubble survey.

The team’s results are being presented today at the American Astronomical Society meeting in Austin, Texas. A paper describing the finding will be published in The Astrophysical Journal.

The telescope spied the stars within 2,600 light-years of the core. After analyzing the stars for nearly a year, Dalcanton’s team determined that they were well past their prime. “The stars are dimmer and have a range of surface temperatures different from the extremely bright stars we see in the star-forming regions of Andromeda,” said Phil Rosenfield of the University of Washington, the paper’s lead author.

As these stars evolved, puffing up to become red giants, they ejected most of their outer layers to expose their blue-hot cores. When normal sun-like stars swell up to become red giants, they lose much less material and therefore never look as bright in the ultraviolet.

“We caught these stars when they’re the brightest, just before they become white dwarfs,” said team member Leo Girardi of the National Institute for Astrophysics’s Astronomical Observatory of Padua. “It is likely that there are many other similarly hot stars in this central part of Andromeda at earlier stages of their lives. But such stars are too dim for Hubble to see because they’re mixed in with a crowd of normal stars.”

The astronomers have proposed two possible scenarios to explain why these blue stars evolve differently. According to Rosenfield, the most likely scenario is that the stars are rich in chemical elements other than hydrogen and helium. Observations with ground-based telescopes have shown the stars in the galaxy’s hub have an abundant supply of “heavy elements,” which makes it easier for stars to eject lots of material into space late in life.

In this scenario radiation from the star is more efficient at pushing on gas laced with heavy elements, which drives away the material, like wind moving a thick sail. Although all the stars in the core are enriched in heavy elements, the bright blue stars may contain especially high amounts, which help trigger the mass loss.

The study also shows that the number of blue stars decreases with distance from the core, tracing the drop in the amount of heavy elements.

Another possible explanation is that the blue stars are in close binary systems and have lost mass to their partners. This mass loss would expose the stars’ hot cores. The astronomers were surprised to find that the ultra-blue stars are distributed in the galaxy in the same way as a population of binary stars with similar masses that were found in X-ray observations by NASA’s Chandra X-ray Observatory.

The astronomers’ next step is to create simulations of these stars to try to determine which scenario is the one that leads them on a different evolutionary path.

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute (STScI) conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.

Image Credit: NASA ESA B. Williams and J. Dalcanton, University of Washington

6 sights to see in the Andromeda Galaxy with a telescope

Start your journey into M31 by looking for the dark dust lane that runs along the northwest edge of the core. Look for the faint glow of the spiral arms beyond this lane.

This glow continues toward another dark lane located further out. Notice also how the central region fades inwards towards a star-like core

Observe how far M31’s core extends from the galaxy’s centre. One measure for this is satellite galaxy M32 which sits 24 arcminutes south of M31’s star-like core.

This mag. +8.1 elliptical appears like a large fuzzy star at low powers. M31’s elliptical core should extend, along its main axis beyond M32. If it reaches the 7th magnitude star HIP 3293, 13 arcminutes southwest of M32, you’re doing well.

M32 itself appears non-circular, an oval glow measuring 3×2 arcminutes. Like M31, the core of M32 appears almost stellar in nature, but larger apertures will reveal it as an extended region about 10-15 arcseconds across. M32 is an elliptical dwarf galaxy with a mass equal to around 3 billion Suns.

Like M32, M110 is another gravitationally bound satellite galaxy of M32 and another elliptical galaxy. It appears fainter and more elongated than M32, M32 being classed as type E2 while M110 is type E6p. M110 sits 35 arcminutes northwest of the centre of M31, 1.5x the apparent distance of M32 from the centre of M31’s core.

The surface brightness of this mag. +8.5 galaxy is lower than M32’s and it can be lost due to light pollution. It has an apparent size about 10×3 arcminutes, appearing like a north-south aligned streak.

NGC 206

We return to the main galaxy for our next object, NGC 206, a bright star cloud. When observing the Andromeda Galaxy through a scope, it is obvious that the main galaxy is dominated by its bright core.

And it takes patience to see anything other than the core. The dark dust lanes mentioned previously are obvious candidates but there are other parts of the galaxy to see.

The trick to finding NGC 206 is to use the obvious visible components – the centre of M31 and M32. The star cloud lies at one vertex of a squat isosceles triangle formed using M32 and M31’s star-like core, M32 being the apex of the triangle. NGC 206 is truly a part of M31.

NGC 185

M32 and M110 are often cited as M31’s satellite galaxies, but they’re not the only ones. NGC 185 is another example. To see it you’ll need to move into Cassiopeia and head 7° north of M31 to arrive at mag. +4.5 Omicron ( ο ) Cassiopeiae. Mag. +9.2 NGC 185 sits 1° to Omicron’s west.

This lesser-known satellite of M31 is a moderately bright dwarf spheroidal galaxy. A 150mm scope will reveal it as about 4 arcminutes across, a bit elongated in an east-west direction. It appears 25% larger in a 250mm scope, with a more circular appearance

NGC 147

NGC 147, is a tricky object, even with large instruments. Head west from NGC 185 for 1°, nudging a little north. This is another dwarf spheroidal galaxy and another M31 satellite.

A 300mm instrument will show it as a faint smudge, 3×2 arcminutes in size, appearing to brighten as you head into the centre towards a stellar nucleus. Like M31, it’s around 2.5 million lightyears from us.

This guide originally appeared in the December 2019 issue ofBBC Sky at Night Magazine. Pete Lawrence is an experienced astronomer and a co-presenter ofThe Sky at Night.

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