Astronomy

How can I identify a star?

How can I identify a star?


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

I'm obviously a novice. I see a star in the eastern sky every night. It is quite large and impressive. How can I identify it. Thank you.


If an astronomical object appears to be very bright, and does not appear to 'twinkle' very much, then it is probably a planet, rather than a star. Jupiter is almost on the opposite side of the Earth to the Sun right now, so it will rise in the East when the Sun is setting in the West. Since Jupiter does look like a very bright star to the naked eye, and since it is in the part of the sky that you describe, then the object that you are seeing is almost certainly Jupiter.

As for recognising actual stars, you really need to learn the layout of the constellations to some extent. This will allow you to find your way around the night sky. A useful tool for this is a program like Stellarium. This will show you the positions of stars and other astronomical objects, including the planets.


The best way to tell the difference between planets and stars is: (From here) Early astronomers were able to tell the difference between planets and stars because planets in our Solar System appear to move in complicated paths across the sky, but stars don't.

That is, if you observe the sky night after night, the stars will all appear in fixed positions with respect to each other. They will rise and set a few minutes earlier each night (an effect that is due to the Earth's motion around the Sun), but otherwise nothing will change. This is why the background stars are sometimes referred to as the "celestial sphere" -- from our point of view, it looks like the stars are "painted" onto a gigantic sphere that surrounds Earth and therefore are unable to move with respect to each other.

Planets, on the other hand, are observed to move in very complicated paths with respect to the background stars, sometimes even appearing to go "against the grain" and reverse their directions. Therefore, they are easily distinguishable from stars if you look at the sky night after night. Although ancient astronomers did not have a correct explanation for this phenomenon, we now know that the complicated motion is just a projection effect -- it is due to the fact that Earth and the other planets are physically moving in orbits around the Sun, so the planets' relative positions as seen from Earth (with respect to the fixed background stars) change as time goes on.

There are other observational differences between planets and stars too, by the way -- such as the fact that planets almost never twinkle.

Like the text said, if an object doesn't twinkle, it's most likely to be a planet. If you observe it every night, and it appears to change its position more than stars do. Although stars also change their positions, they don't change their positions in the sky as much as planets throughout the year.

Stars maintain positions relative to one another, constellations are a great to tell the difference between planets in stars. What I mean by maintaining their positions is that each night they are usually in the same pattern and usually the same distance apart in the night sky.


Astronomers Identify The Star Systems That Could Be Watching Earth From Space

If there were alien civilizations in other star systems, would they be able to detect our presence here on Earth? This is a question that could lead us to new ways to search for signs of extraterrestrial intelligence, but not one that is necessarily easy to answer.

Nevertheless, a team of astronomers has identified 2,034 star systems within 100 parsecs (326 light-years) of Earth that would have the right vantage point to detect Earth life signs as our home planet orbits the Sun.

"From the exoplanets' point of view, we are the aliens," said astronomer Lisa Kaltenegger of Cornell University's Carl Sagan Institute.

Using data from the Gaia space observatory - an ongoing project to map the Milky Way in three dimensions with the highest precision yet - Kaltenegger and her colleagues sought to determine if any alien civilizations out there could find humanity with the tools we use to find exoplanets.

We have several of these, but the most fruitful technique is what we call the transit method. When an exoplanet orbits a star, if that orbit is aligned just right, it will pass between us and its host star, known as a transit. This creates a very specific light curve signature as the starlight dims and brightens fractionally due to the exoplanet's transit.

We can tell the size of the exoplanet, roughly, by the depth of the light curve, which can help rule out exoplanets unlikely to host life as we know it, such as gas giants like Jupiter.

Additionally, if the exoplanet has an atmosphere, astronomers can stack transits to amplify the spectrum of the host star's light that passes through it. The way some wavelengths are enhanced or absorbed by the atmosphere can reveal its composition - including gases that indicate signs of life.

If, like Earth, those exoplanets had developed technology that pollutes its atmosphere, then hypothetically, we might be able to detect that too (although we haven't yet).

The Gaia data has allowed Kaltenegger and her team to search for star systems that may have been able to do the same to us, over a 10,000-year span: from 5,000 years in the past up to 5,000 years into the future. This vantage point is known as the Earth Transit Zone.

"We wanted to know which stars have the right vantage point to see Earth, as it blocks the Sun's light," she said. "And because stars move in our dynamic cosmos, this vantage point is gained and lost."

According to their analysis of the Gaia data, the researchers worked out that there have been 1,715 star systems in the Earth Transit Zone in the last 5,000 years that could have detected biosignatures as human technological civilizations emerged. An additional 319 star systems will enter the Earth Transit Zone in the next 5,000 years."Gaia has provided us with a precise map of the Milky Way galaxy, allowing us to look backward and forward in time, and to see where stars had been located and where they are going," said astrophysicist Jackie Faherty of the American Museum of Natural History.

Of the systems that have been or will be in the Earth Transit Zone, seven are known to host exoplanets, some of which may even be habitable. They include Ross-128, TRAPPIST-1, and Teegarden's star.

Ross-128 had in the past a 2,158-year span in the Earth Transit Zone. TRAPPIST-1 will enter it in about 1,642 years, and stay there for another 2,371 years. Teegarden's star, due to enter the zone in 29 years' time, will have a much smaller window of just 410 years.

"Our analysis shows that even the closest stars generally spend more than 1,000 years at a vantage point where they can see Earth transit," Kaltenegger said. "If we assume the reverse to be true, that provides a healthy timeline for nominal civilizations to identify Earth as an interesting planet."

As part of their research, the team also had a look at which stars would be able to detect technosignatures - the technogenic radiation emitted from Earth. We first started transmitting radio waves into space only about 100 years ago, which means there's roughly a 100 light-year radius around us from which those signals could be detected.

Within that radius, and that 100-year timeframe, there have been 75 systems in the Earth Transit Zone.

Our searches for alien civilizations have so far revealed no signs. The team's research demonstrates that there are significant limitations for our detection methods, not least the ever-evolving configurations of the stars around us. But, given enough time, and luck, it may be possible to locate our cosmic neighbors.


Identifying star clusters

The Pleiades, the Hyades and the Beehive cluster are all amazing in their own right, but, to pose a straightfoward question: which is which?

Imagine that you're separated from any kind of star chart. So how, other than searching by Right Ascension (RA) or Declination (DEC), could one know where to look?

The Pleiades is also known by its Messier catalogue number M45. Although nicknamed the Seven Sisters (map identifying stars of the Pleiades), M45 is actually formed by over 100 stars.

It's located at RA 3:47, DEC +24.07 with a visual brightness of 1.6, and apparent dimension of 110.0 arcminutes.

The Hyades, also known as Melotte 25, can be found at RA 4:27, DEC +16 with a visual brightness .5 magnitude and apparent dimension of 330 arcminutes.

Both the Pleiades and the Hyades are open star clusters in the constellation Taurus (the Bull) and can been seen with the naked eye.

Locate the Seven Sisters perched on the Bull's right shoulder. They lie about 4 degrees from the ecliptic so are frequently occulted by the Moon and other planets (which proves to be great observer eye candy!). There is quite a bit of nebulosity located within the Pleiades, especially around the brightest stars.

I enjoyed watching them appear to sit on the mountain summit where I lived outside Mexico City. Greek mythology relates the story of the Pleiades placed in the sky while seeking refuge from Orion's nonstop pursuit.

The Hyades, 150 light-years away from Earth, outlines the Bull's face and is a loose V-shaped cluster of white stars. It's less populated and younger than the Pleiades. While the central star group is about 10 light-years in diameter, the outer group is spread over 80 light-years. Astronomical studies of the sky show the Hyades moving eastward in the sky toward Betelgeuse in Orion.

It should be noted that the bright red star, Aldebaran (Alpha Tauri) - the eye of the Bull - is not a member of this cluster despite lying in the Hyades field.

It is thought that the Hyades may have a common origin with the Beehive cluster (located in the constellation Cancer), due to their similarities in proper motion and age.

The Beehive cluster lies at RA 08 : 40.1, DEC +19 : 59, with a visual brightness of 3.7 magnitude and apparent dimension of 95.0 arcminutes. Known also as Praesepe (Latin for manger), it's Messier No. is M44. When observing it, look for the eclipsing binary star TX Cancri. Epsilon Cancri is also an eye catcher and worth spending some time to find.


The Myth: Queen Cassiopeia of Ethiopia

In Greek mythology, Cassiopeia was the wife of King Cepheus of Ethiopia. The vain queen boasted that she or her daughter (accounts vary) were more beautiful than the Nereids, sea nymph daughters of the sea god Nereus. Nereus took the insult to the god of the sea, Poseidon, who rained his wrath down upon Ethiopia. To save their kingdom, Cepheus and Cassiopeia sought the counsel of the Oracle of Apollo. The oracle told them the only way to appease Poseidon was to sacrifice their daughter, Andromeda.

Andromeda was chained to a rock near the sea, to be devoured by the sea monster Cetus. However, the hero Perseus, fresh from beheading the Gorgon Medusa, saved Andromeda and took her as his wife. At the wedding, Perseus killed Andromeda's betrothed (her uncle Phineus).

After their deaths, the gods placed members of the royal family near each other in the heavens. Cepheus is to the north and west of Cassiopeia. Andromeda is to the south and west. Perseus is to the southeast.

As punishment for her vanity, Cassiopeia is forever chained to a throne. However, other depictions show Cassiopeia on a throne unchained, holding a mirror or palm frond.


Learn Astronomy HQ

As astronomers, we're always delighted to see a meteorite streak across the night-sky while we're setting up our scopes or scanning the constellations for our next setup. At other times, we're carefully observing an area of the sky in anticipation of a meteor shower. On some rare occasions we are left wondering in amazement as a meteorite streaks down and over the horizon. Where did it land? We wonder. And will someone find it?

The answer is that thousands of meteorites land on the Earth's surface each year. In fact, according to The Monthly Notices of the Royal Astronomical Society, 18,000 to 84,000 meteorites strike the Earth each year, although other estimates put the total annual number as low as 500. To date, it's reported that 40,000 meteorites have been found. That sounds like a small number, but once again there aren't that many people who take the time to look for them.

You Can Do It

Most meteorites that we might find are smaller pieces of larger meteoroids that have broken up as they travel through Earth's atmosphere. The fact is they're out there and not that many people are looking for them so you have a good chance of discovering one if you know how and where to look, and what to look for.

One Indispensable Tool - The Neodymium Magnet

Neodymium Magnet

The one, simple tool every meteorite hunter uses is something called a "Neodymium" magnet attached to the end of a stick or cane. Neodymium magnets attract rare earth metals in addition to the iron that is common in many meteorites. You must be careful though as these magnets are very strong. So keep them out of reach of children and don’t let two attract to each other as they could shatter.

Types of Meteorites

For the record, meteorites typically occur in 3 types.

1. The iron meteorite - consisting mostly of iron. These are the meteorites attracted to the Neodymium magnet, but only 5% of meteorites that reach the earth are of this type.

2. The stony meteorite - or "chondrite" is made up of the rocky material from its source. They often have circular inclusions or "chondrules" from the gases that escaped in the heat of entry and can be any size from fist sized to the smallest pebble. Stony meteorites could be from a planet or asteroid, or pieces flung into space as a result of a planet/asteroid collision. 80 to 95% of the meteorites that fall to Earth are rocky meteorites.

3. The stony/iron meteorite - these usually consist of a 50/50 mix of iron and silicates. They are also very heavy for their size. Only 1 to 5% of falls have this stony/iron combination.

What's important to remember is that a rock with a stone-like appearance that is attracted to a magnet increases the odds that you may have found a meteorite.

What Are the Odds of Finding One Yourself?

While it may seem you're more likely to find a rocky meteorite given the 95% fall-rate, 80% of the finds are metallic, while only 20% are pure rock. This is due to the ease of finding an iron meteorite with a Neodymium magnet, the fact that metallic meteorites don't erode as quickly as rocky meteorites, and the striking appearance of most iron/metallic meteorites.

In fact, many meteorite hunters don't know a rocky meteorite when they find them, or mistake an unusual rock of Earth origin for a meteorite. The common term for this mis-identification is a "meteorwrong."

Tool Number 2 - A Field Guide to Meteorite Identification

The second tool you might want to consider is either a book that shows you the appearance and types of meteorites, or a club where experienced meteorite hunters can give you some tips and advice. Most look for any magnetic properties, a black, fused outer surface that would indicate some level of exposure to high heat upon entry into the atmosphere, and in some instances a weight that seems greater than you would expect for a rock that size. Be mindful of the fact that after some meteorites have weathered with time the colour can vary from black to either brown, yellow, orange, or a reddish appearance. There are other tests that include sawing a sample in half with a diamond saw to look for a unique pattern in the interior, but that will spoil the sample.

Rust on the exterior of a sample can also indicate a meteorite find, but be careful. Hematite and Magnetite also rust and are attracted to magnets. The easiest way to know is do a streak test. Hematite and Magnetite will both leave a coloured streak on rough piece of porcelain or ceramic or on a geologist's streak stone. Hematite will leave a rust coloured streak and Magnetite a dark, gray streak. Meteorites leave no streak unless you press extremely hard and it will usually be a very light, grayish colour.

Where to Look

Unfortunately, knowing what they look like and simply assembling tools for a meteorite hunt won't guarantee you'll find them. You need to know where to look.

Some of the best meteorite hunting grounds are dry lake-beds any large, barren expanses where there are few terrestrial rocks deserts, icy regions (the most meteorites have been found in Antarctica), or something called "strewn-fields."

Strewn-fields are zones where meteorites from a single space rock were dispersed as it broke into pieces as it exploded in Earth's atmosphere during entry. Recently, many small pieces of the meteorite that exploded over Chelyabinsk, Russia are being found by people living in the area even when they weren't looking for them. The strewn-field is enormous.

To locate potential strewn-fields in your area you can check with the local conservation office, or search the internet. Even if you can't find a strewn field and deserts aren't particularly common in your area, any walk across open ground with your Neodymium magnet stick might surprise you.

If and when you do find success you might get the bug to continue your meteorite hunts. In that case you might want to add some tools for your expeditions.

Tool Number 3 - GPS and Notebook

A handheld GPS and notebook is a good addition. If you find a meteorite you can identify the position. Who knows, maybe you've found a new strewn-field no one knows about. It also helps to remind you where you've hunted in the past so you don't cover the same ground again.


SkyView‪®‬ 4+

SkyView® brings stargazing to everyone. Simply point your iPhone, iPad, or iPod at the sky to identify stars, constellations, planets, satellites, and more!

Over 2.5 million downloads.

App Store Rewind 2011 -- Best Education App

“If you've ever wanted to know what you're looking at in the night sky, this app is the perfect stargazer's companion.”
– CNET

"If you’ve ever been looking for a stargazing app for your iPhone, then this [is] definitely the one to get."
– AppAdvice

“SkyView is an Augmented Reality app that lets you see just what delights the sky has to offer.”
– 148Apps Editor’s Choice

You don't need to be an astronomer to find stars or constellations in the sky, just open SkyView® and let it guide you to their location and identify them. SkyView® is a beautiful and intuitive stargazing app that uses your camera to precisely spot and identify celestial objects in sky, day or night. Find all 88 constellations as they fade in and out while you scan across the sky, locate every planet in our solar system, discover distant galaxies, and witness satellite fly-bys.

• Simple: Point your device at the sky to identify galaxies, stars, constellations, planets, and satellites (including the ISS and Hubble) passing overhead at your location.
• Sighting Events: schedule alerts for upcoming celestial events.
• Apple Watch: see what objects are visible tonight and how to spot them using your Apple Watch.
• Today Widget: brand new iOS 8 today widget lets you quickly see upcoming celestial events.
• Night Mode: Preserve your night vision with red or green night mode filters.
• Augmented Reality (AR): Use your camera to spot objects in the sky, day or night.
• Sky Paths: Follow the sky track for any object to see it’s exact location in the sky on any date and time.
• Comprehensive: Includes thousands of stars, planets, and satellites with thousands of interesting facts.
• Time Travel: Jump to the future or the past and see the sky on different dates and times.
• Social: Capture and share beautiful images with friends and family on social networks.
• Mobile: WiFi is NOT required (does not require a data signal or GPS to function). Take it camping, boating, or even flying!

What a fun way to teach yourself, your children, your students, or your friends about our wonderful universe!


Learn Astronomy HQ

Shaped like a rose in the night sky. This incredible nebula sits around 1000 light years away but still is visible to the naked eye. The nebula is a nursery where new stars are being formed.

Messier 42, the Orion nebula has to be one of the most breathtaking deep space objects to see in the night sky. It resides 1,345 light years away and was first discovered in 1610 by Fabri de Peiresc. This diffuse nebula spans an area some 20,000 times the size of our solar system and is the nearest source of intense star formation to us. It is visible to the naked eye with a magnitude of 4, but M42 is so large it has a surface brightness of 11. The distribution of its light isn't uniform across the object and is mainly in the centre making this a deceiving surface brightness, as the object is much closer to magnitude 4 than 11. When looking at the Orion nebula try to observe the four stars at its centre that make up the Trapezium. You'll probably need to increase your magnification to resolve all four of them. Enjoy this rose shaped beautiful and bright nebula.

Want more help? Get this book

Other Names:

Telescope Image to Scale:

The nebula is so large that the image above is scaled at 120 x 120 arc minutes. The full moon is equivalent to 30 x 30 arc minutes.

The surface brightness is confusing here. It suggests the nebula is quite faint. This is because a large part of the nebula is very faint but the centre is very bright. The surface brightness is a measure of how bright the object would be if all the light from it was spread evenly over the whole object. As a large part of the nebula is very faint this brings the surface brightness number to 11.

Telescope Aperture City Suburbs Rural Dark Sky
4 inches 9 10 11 12.5
5 inches 9.5 10.5 11.5 13
6 inches 10 11 12 13.5
8 inches 10.5 11.5 12.5 14
10 inches 11 12 13 14.5

Use this chart to estimate if you can see the object from your location with your telescope. For more help on using this chart click here.

How to See It:

This is one of the easiest nebula to see. It is visible to the naked eye on a dark night. It can be found below Orion's belt, hanging like a sword. It's co-ordinates are: RA 05 h 36 m 05.15 DEC -05° 26′ 37.1″ if you are using guiding software.

The nebula is easily visible in binoculars and small telescopes. The trick is to see if you can resolve the stars in the shape of a trapezium in the middle of the nebula. There are actually five stars in the trapezium, but you would struggle to see this with small telescope. Below is the shape of stars you are looking for.

Credit: NASA, C.R. O'Dell and S.K. Wong (Rice University)

To find more space objects have a look at the constellation guide by clicking the link or go back to the list of deep space objects.


By Phil Harrington, Jake Parks

Many people who live in a light-polluted city and are interested in astronomy never buy a telescope — and that’s simply because they can’t see the stars. Runaway light pollution is robbing us of the night sky. But take heed all is not lost. Many amateur astronomers who live in and around a light-polluted city still enjoy wonderful telescopic sights during clear evening nights. And you can too you just have to know a few tips and tricks to achieve the best your equipment has to offer.

But first, we need to define a couple terms. Light pollution comes in two varieties: sky glow and local, or line-of-sight, glare. Sky glow refers to the sickish orange glow produced by city lights that we see rising above the horizon, extinguishing faint stars and other celestial objects. Localized, or line-of-sight, light pollution, however, can happen anywhere. Most often, it’s caused by one or more poorly designed or placed lights, either from a neighbor’s house or a poorly located streetlight. By itself, line-of-sight light pollution may not wash out the sky like sky glow does, but it can blind an observer to faint starlight by sneaking into the corner of their eye.

The good news is that with these top tips and tricks, it is possible to observe the night sky in a light-polluted city.

Large or small telescopes for urban observing?

Large telescopes suffer more from sky glow, but they also gather more starlight, regardless of where they’re located. A 12-inch telescope will always show you more stars than a 6-inch scope.

However, the bottom line is that unless a telescope is convenient to use, it will quickly become relegated to a dark corner. In other words, light pollution should have no impact when choosing a telescope’s aperture, but your available storage space should be at the top of the list.

How to get your telescope ready for city observing

Urban astronomers greatly benefit from computerized telescope mounts. While star-hopping is fantastic for locating targets from a dark-sky site, it’s hard to do if the sky is so bright that reference stars elude you. Plus, because cheap computerized telescopes (with a mount included) often divide their strengths between electronics and optics, they usually compromise on both. Therefore, in most cases, your best bet is probably to purchase a separate computerized mount to complement your scope.

Choosing the right eyepiece is just as important as choosing the right telescope. The eyepiece can make all the difference between seeing an object and not. Never select an eyepiece based on the magnification it will deliver through a given telescope. When observing under less than ideal conditions, magnification plays second fiddle to the exit pupil — or the diameter of the beam of light that leaves the eyepiece. (The exit pupil’s diameter will change as magnification goes up and down.) Knowing the diameter of the exit pupil is critical, because if it’s too large or too small, the resulting image may prove unsatisfactory. To find out its diameter, use one of the following two formulas:

Exit pupil = diameter of the telescope’s objective lens (primary) in millimeters [divided by] magnification.

Exit pupil = focal length of the eyepiece in millimeters [divided by] telescope’s focal ratio, or its “f/ number.”

The significance of the exit pupil is especially important under light-polluted skies, which leads to much lower contrast between celestial objects and the background. If an eyepiece produces too large an exit pupil, contrast will suffer greatly. On the other hand, if the eyepiece produces too small of an exit pupil, the images may be so dim they are nearly impossible to see. With that said, there is no single best exit pupil for viewing all objects. The table below offers suggestions for urban conditions.

For urban observing conditions, you want an exit pupil diameter of 3 to 5mm for large star clusters and the full lunar disk 2 to 4mm for small deep-sky objects like planetary nebulae, small galaxies, double stars, lunar details, and planets on nights of poor seeing and 0.5 to 2mm for double stars, lunar details, and planets on nights of exceptional seeing.

Telescope filters are also a great way to better view planets. Color filters allow backyard astronomers to see certain features easier because they exaggerate brightness differences (contrast).


How to find Orion

The great thing about the constellation of Orion is that it’s so easy to spot, as long as you’re looking at the right time. It’s a winter constellation (a star pattern), and during the 100 Hours of Astronomy you can see it at any time from before 6 pm until the early hours of the morning. You’ll know it when you see it, because there are three quite bright stars in a line, quite close together and evenly spaced. They are at the centre of a quadrilateral of stars, including two of the brightest in the sky.

Orion over rooftops. Photo: Robin Scagell

Where to look

If you’re looking during mid January in the evening, Orion will be rising over in the east at around 6 pm, or later in the evening it will be more to the south, and at a different angle, as these examples show:

Orion rising in the east about 6 pm in mid January

Orion due south about 10 pm in mid January

Later still in the evening Orion will be tilting over to the west. But you’ll still be able to recognise its shape. The three stars in a line are known as Orion’s Belt, for reasons that date back possibly thousands of years, when most of the major constellations were first described. The patterns that we see in the sky are virtually the same as they saw then, except that with no light pollution people would have seen them much more easily. People would have told tales around the camp fire just as we watch stories on the TV today. But with no books or writing available, they would have used the patterns of stars themselves as a story board.

So it is with Orion, the Hunter. Many different civilisations saw this pattern as some sort of hunter or giant, though the name Orion dates from ancient Greek times. He is usually shown facing to the right, facing the constellation of Taurus, the Bull, with an upraised shield with a lion’s head marked by a curved line of stars, and a raised club in his other hand.

Constellation figures for Canis Major, Lepus, Orion and Taurus superimposed on the stars. Artwork by Richard Bizley

And here’s a map of the whole sky for 12 January at 8 pm. Because it’s designed for looking above you rather than at the ground, east and west are reversed compared with a conventional map.

Map created using Stellarium software

Find the Orion Nebula

Once you’ve found Orion itself, look for the Orion Nebula. This is below the thee stars that mark Orion’s Belt, and in a dark sky you can see it with the naked eye. But even from a light-polluted location you can get a glimpse of it using binoculars. But don’t expect to see the glorious colours that you see in photos – our eyes just aren’t sensitive enough to see them.

The three stars of Orion’s Belt are your guide to locating the Orion Nebula. Photo: Robin Scagell

The photo shows roughly what you’ll see through binoculars. The nebula itself is fainter than it shows in the photo, but you should be able to see that the middle star of the three that hang down from the Belt of Orion is slightly hazy.

This area is the birthplace of stars, and all the bright stars of Orion (with the exception of Betelgeuse at top left) are very bright and young stars that have been born in this area within the past few million of years. But don’t expect to see a new star suddenly pop into existence – the process takes hundreds of thousands of years. Here’s a close-up photo made with a telescope:

The Orion Nebula (catalogue number M42) as photographed through a 200 mm amateur telescope. Photo: Robin Scagell

Star colours

Compare the colours of the bright stars at top left and bottom right of the pattern, Betelgeuse and Rigel. Can you spot the difference? Betelgeuse is noticeably redder than Rigel. This difference is due to the temperatures of the two stars. Betelgeuse is what’s known as a red supergiant, and although it has swollen up to giant size, its temperature is comparatively low, about 3500 K (K means Kelvin, the scientific temperature scale which is like Celsius but which starts from absolute zero, about –273º C). This is about the same temperature as a halogen light bulb. Rigel, on the other hand, has a temperature of around 12,000 K.
Betelgeuse is in the last stages of its life as a bright star. In the not-too-distant future it will become a supernova, and will outshine all the other stars in the sky, and become visible even in daylight. Then after a few months it will fade away to obscurity. This could happen at any time – any time, that is, within the next million years or so. It could even happen during the 100 Hours of Astronomy – but then again, it probably won’t!

Compare the colours of Betelgeuse and Rigel


How Do We Classify The Stars In The Universe?

The stars found in NGC 3532 show a rich variety of colors and brightnesses. Image credit: ESO/G. . [+] Beccari.

Take a look up at a dark night sky, and you'll find it illuminated by hundreds or even thousands of individual twinkling points of light. While they might seem, to an untrained eye, to all be the same -- except for, perhaps, some appearing brighter than others -- a closer look reveals a number of intrinsic differences between them. Some of them appear redder or bluer than others some are intrinsically brighter or fainter, even if they're the same distance away some have larger physical sizes than others some have greater or lesser percentages of heavy elements in them. For a long time, scientists didn't know how stars worked or what made one type different from another. Yet at the start of the 20th century, the pieces all came together to figure out exactly how the different stars should be classified, and we owe it all to a woman you might not have heard of: Annie Jump Cannon.

Annie Jump Cannon sitting at her desk at Harvard College Observatory, sometime in the early 20th . [+] century. Image credit: Smithsonian Institution from the United States.

With either good enough skies and a trained observer, or with a quality telescope, a look at the stars immediately shows that they come in different colors. Because temperature and color are so closely related -- heat something up and it glows red, then orange, then yellow, white and eventually blue as you turn up the temperature -- it makes sense that you'd classify them based on color. But where would you make those divisions, and would those divisions encapsulate all the important physics and astrophysics going on? Without more information, there wouldn't be a good, universal system that everyone would agree on. But the study of color in astronomy (photometry) can be augmented by breaking up the light into individual wavelengths (spectroscopy). If there are either neutral or ionized atoms in the outermost layers of the star, they'll absorb some of the light at particular wavelengths. These absorption features can add an extra layer of information, and led to the earliest useful classification system.

The solar spectrum shows a significant number of features, each corresponding to absorption . [+] properties of a unique element in the periodic table. Image credit: Nigel A. Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF.

Known as Secchi classes, for the 19th century Italian astronomer Angelo Secchi who devised them, there were originally three types:

  1. Class I: a class for the blue/white stars that exhibited strong, broad hydrogen lines.
  2. Class II: yellow stars with weaker hydrogen features, but with evidence of rich, metallic lines.
  3. Class III: red stars with complex spectra, with huge sets of absorption features.

This system, first laid out in 1866, was the first non-arbitrary system of classification, since it relied on a combination of spectroscopic features in tandem with the photometric colors. While Secchi went on to further refine his class structure and introduce sub-classes and additional classes, this became superseded by finer spectral delineations.

The original three Secchi classes, and the accompanying spectra that go along with them. Image . [+] credit: from a colored lithograph in a book published around 1870, retrieved from AIP.

Researchers at Harvard College Observatory were tasked with surveying all the stars visible in the night sky down to a visual magnitude of +9, or the faintest you'd be able to see today with a very nice pair of binoculars. Except it wasn't enough to record them in the traditional fashion they needed to be observed and analyzed spectroscopically. Under the guidance of Edward Pickering, a group of astronomers -- all women, known at the time as "Pickering's Harem" (that was later sanitized to "Pickering's Women" or the "Harvard Computers") -- took the data and created the Draper System, for which Pickering was given sole/full credit. The stars that had the strong hydrogen lines (Secchi Class I) were broken up into four further delineations, labeled A through D, based on how strong the hydrogen absorption features were, with A being the strongest. The stars with rich, metallic lines (and weaker hydrogen lines, Secchi Class II) were broken up into six classes, E through L, with decreasing hydrogen strength and increasing metal strength going hand-in-hand. The reddest stars, richest in absorption features (Secchi Class III) became class M. In addition, there were four other types labeled N through Q, with O being notable as having very bright, blue stars with very weak hydrogen features, but also lines not seen in any other star class.

The seven major star classes, organized by their colors. It turns out that these colors also . [+] correspond to a star’s surface temperature, and so O-stars are the hottest, while M-stars are the coolest. Image credit: E. Siegel.

In 1901, Annie Jump Cannon -- one of the astronomers working under Pickering -- synthesized the full suite of this data and consolidated the seventeen Draper System classes into just seven: A, B, F, G, K, M, and O. The big step that she took, however, was also perhaps the simplest: to reorder them by their color, from bluest to reddest. This meant the order was now O, B, A, F, G, K, and M. Star types were further broken down into ten intervals apiece, from 0 to 9, based on bluest to reddest. So a B2 star would be 20% of the way between a B0 star and an A0 star, a B5 star would be 50% of the way there, and a B9 star would be 90% of the way there. The bluest star of all would be O0, while the reddest would be M9. This system, known as the Harvard Spectral Classification System, is still in use today. There would, however, be one more great leap that would happen decades after Annie Jump Cannon's contributions, and you can see it for yourself if you view the spectra of these different classes in descending order.

O-stars, the hottest of all stars, actually have weaker absorption lines in many cases, because the . [+] surface temperatures are great enough that most of the atoms at its surface are at too great of an energy to display the characteristic atomic transitions that result in absorption. Image credit: NOAO/AURA/NSF, modified to illustrate the stars that demonstrate this phenomenon.

You'll notice that certain lines appear, get stronger and then disappear, while others simply appear and strengthen. The reason stars appear with the absorption features they do are because of their temperature, and because at certain temperatures different ionization states (and hence, different atomic transitions) are more common, and therefore, stronger. The link between temperature, color and ionization wasn't found until 1925, with the Ph.D. dissertation of Cecilia Payne, which also enabled us to determine what the Sun (and all stars) were actually made out of! The different stellar classifications don't just correspond to a star's colors and absorption features, but to a star's temperature as well.

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star . [+] class shown above it, in kelvin. Image credit: Wikimedia Commons user LucasVB, additions by E. Siegel.

Thanks to Payne and Cannon's work, we learned that stars were made out of mostly hydrogen and helium, and not out of heavier elements like Earth is. Cecilia Payne's work would have been impossible without Annie Jump Cannon's data Cannon herself was responsible for classifying, by hand, more stars in a lifetime than anyone else: around 350,000. She could classify a single star, fully, in approximately 20 seconds, and used a magnifying glass for the majority of the (faint) stars. Her legacy is now nearly 100 years old: on May 9, 1922, the International Astronomical Union formally adopted Annie Jump Cannon's stellar classification system. With only minor changes having been made in the 94 years since, it is still the primary system in use today.