# Will it be dangerous when close stars become Red Giants?

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What will happen if close stars like Sirius A or Procyon_A become red giants? Is it dangerous for the Earth and life on it?

If for Sirius A we still have 500-800 million years, but Procyon A might become a red giant in 10 to 100 million years, by astronomical scale it is very soon.

Or nothing will happen with the Earth, because it is still too far?

Life on Earth will not be adversely affected when nearby stars become red giants.

Certainly, things will get very messy inside those systems: red giants throw out a lot of gas and dust. Red giants are a lot more luminous than the Sun because they have such a huge surface area, but they don't emit large amounts of dangerous radiation, like X-rays or ultraviolet light. In fact, a red giant is cooler than the Sun, so it emits relatively little UV light.

According to Wikipedia, it's possible that a red giant system could even be habitable, so life in nearby systems should be quite safe.

Although traditionally it has been suggested the evolution of a star into a red giant will render its planetary system, if present, uninhabitable, some research suggests that, during the evolution of a 1 M☉ star along the red-giant branch, it could harbor a habitable zone for several billion years at 2 AU out to around 100 million years at 9 AU out, giving perhaps enough time for life to develop on a suitable world. After the red-giant stage, there would for such a star be a habitable zone between 7 and 22 AU for an additional 109 years. Later studies have refined this scenario, showing how for a 1 M☉ star the habitable zone lasts from $$10^8$$ years for a planet with an orbit similar to that of Mars to $$2.1×10^8$$ yr for one that orbits at Saturn's distance to the Sun, the maximum time ($$3.7×10^8$$ yr) corresponding for planets orbiting at the distance of Jupiter. However, for planets orbiting a 0.5 M☉ star in equivalent orbits to those of Jupiter and Saturn they would be in the habitable zone for $$5.8×10^9$$ yr and $$2.1×10^9$$ yr respectively; for stars more massive than the Sun, the times are considerably shorter.

However, the Sirius and Procyon systems mentioned in the question are both binary systems, and both Sirius B and Procyon B are already white dwarfs. That makes the picture more complicated, and definitely more dangerous inside those systems, but neighboring stellar systems won't be harmed.

When Sirius A and Procyon A become red giants their white dwarf companions will get bombarded by a lot of material. If a white dwarf accretes enough hydrogen, it can lead to a runaway fusion reaction, in other words, a nova explosion.

Hydrogen fusion may occur in a stable manner on the surface of the white dwarf for a narrow range of accretion rates, giving rise to a super soft X-ray source, but for most binary system parameters, the hydrogen burning is unstable thermally and rapidly converts a large amount of the hydrogen into other, heavier chemical elements in a runaway reaction, liberating an enormous amount of energy. This blows the remaining gases away from the surface of the white dwarf surface and produces an extremely bright outburst of light.

A nova is bright, but it only emits a tiny amount of energy compared to a supernova, and poses no danger to neighboring stellar systems.

Actually, it's very unlikely that either Sirius B or Procyon B will have nova events: they are simply too far from their companions to accrete enough hydrogen for that to occur. (Thanks to Peter Erwin for that info).

Nothing will happen. Neither star is massive enough to become a supernova and their velocities relative to the Earth mean that we will almost certainly be hundereds of light years away from them when they become red giants.

A 1 km/s velocity difference over 100 million years leads to a distance difference of about 300 light years.

Both Procyon and Sirius are in binary systems with white dwarfs, but they are not close binary systems. It is possible, but unlikely, that their white dwarf companions could accrete sufficient material to cause a Type Ia supernova - a possibility I address in Will Sirius B start accreting from A and become a supernova type Ia? but even then, as I said above, these stars will be nowhere near the Sun when that happens.

## 1 in 10 Red Giants are Covered in Spots, and They Rotate Surprisingly Quickly

Sunspots are common on our Sun. These darker patches are cooler than their surroundings, and they’re caused by spikes in magnetic flux that inhibit convection. Without convection, those areas cool and darken.

Lots of other stars have sunspots, too. But Red Giants (RGs) don’t. Or so astronomers thought.

A new study shows that some RGs do have spots, and that they rotate faster than thought.

The new study is titled “Active red giants: Close binaries versus single rapid rotators.” Lead author is Dr. Patrick Gaulme from the Max Planck Institute. The paper is published in the journal Astronomy and Astrophysics.

Red Giant stars are at a late stage of stellar evolution. All stars rotate, but as RGs lose mass and expand, that rotation slows down, like a figure skater who stretches out their arms. That slower rotation calms the dynamo process inside the star, and that dynamo process is what fuels the star’s magnetic activity. Less magnetic activity means fewer spots.

But this new study finds that some RGs don’t conform to this understanding. The study shows that about eight percent of RGs rotate rapidly and produce starspots.

This image tracks the life of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant star on the right. On the left the star is seen as a protostar, embedded within a dusty disc of material as it forms. It later becomes a star like our Sun. Eventually, it’ll enter a helium-burning phase, expand, and turn red. Our Sun has no binary companion, so likely will not rotate rapidly and produce sunspots anymore. Image Credit: By ESO/M. Kornmesser – http://www.eso.org/public/images/eso1337a/, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=27981948

“Rotation and convection are both crucial ingredients for the formation of surface magnetic fields and starspots,” explains Dr. Federico Spada of MPS, co-author of the new study, in a press release. “Stars with outer convective layers have the potential to generate surface magnetic fields via dynamo action, but only when the star rotates fast enough the magnetic activity becomes detectable,” he adds.

But now there are exceptions to that understanding of RG stars.

Between 2009 and 2013, NASA’s Kepler spacecraft gathered data on about 4,500 red giant stars. The team of researchers behind this study examined all of that data. Though Kepler was designed to ferret out dips in star brightness caused by planets transiting in front of the star, it gathered data on all dips in brightness, including those caused by starspots. RGs typically rotate slowly, so these dips in brightness from starspots rotate in and out of view over the course of a few months. And they repeat.

When the team found that about 8% of the stars are rotating more rapidly than expected, they wondered how the stars got the energy to do so. “To answer this question, we had to determine as many of the stars’ properties as possible and then put together an overall picture,” said lead author Dr. Patrick Gaulme.

Artist’s impression of the structure of a solar-like star and a red giant, not to scale. Image Credit: ESO

The researchers examined a number of things. They looked at the wavelengths of light from the stars and how they changed over time. They looked at rapid fluctuations in the stars’ brightness. Those rapid fluctuations are caused by pressure waves coming from the star’s center to the surface, and they’re superimposed on the dips caused by starspots. The pressure wave fluctuations are like a window into the interior of the star, and they hold clues about the star’s mass and age.

But the most interesting finding was that about 15% of the rapidly rotating RGs with starspots are in close binary relationships with another star, usually one that’s smaller and less massive.

“In such systems, the rotational speeds of both stars synchronize over time until they rotate in unison like a pair of figure skaters,” says Gaulme. So the slowly-rotating Red Giant gains rotational energy from its binary partner. It spins faster than a RG without a binary buddy.

But that doesn’t explain the other 85% of RGs with starspots that are also rotating more rapidly than expected. Those stars don’t have binary companions to gain their rotational energy from. What gives?

“In total, behind the common observational feature that some red giants have spots, we find three groups of rapidly rotating stars, each of which has a very different explanation.”

Dr. Patrick Gaulme, Lead Author, Max Planck Institute

To start with, the researchers divided those stars into two groups: stars that have a mass roughly equal to the Sun, and stars that have two to three times as much mass as the Sun.

The team says that the stars roughly equal to our Sun likely merged with another star, or maybe a planet, and gained rotational energy that way. The stars in the more massive group developed differently. They likely had an internal structure that was quite different from the rest. They may never have created the kind of global magnetic field that, over a long period of time, carries mass away from the star as particles. Without that mass loss, their rotation never slowed, and they rotate more quickly to this day.

The study found that there are three different paths Red Giants can follow to produce starspots. Image Credit: MPS / hormesdesign.de

“In total, behind the common observational feature that some red giants have spots, we find three groups of rapidly rotating stars, each of which has a very different explanation. So it’s no wonder that the phenomenon is more widespread than we previously thought,” said Gaulme.

What does this mean for exoplanets orbiting RGs? The study exposes some of the detail in Red Giant stars, and the complexity that governs their impact on the habitability of any planets around them. But any conclusions or rules that can be applied to them or their planets will have to wait for missions like the ESA’s PLATO (PLAnetary Transits and Oscillations of stars) mission. PLATO will examine the properties of terrestrial exoplanets in the habitable zones around their Sun-like stars. It’ll also study the seismic activity in those stars, further exploring the relationships between habitable zone planets and their stars.

“We look forward to having the PLATO mission in space with its unique long-duration observations we will be able to extend the study to other regions of the Milky Way,” concluded study co-author Spada.

## Stellar Evolution

Stars are not static objects. As a star consumes fuel in its nuclear reactions, its structure and composition changes, affecting its color and luminosity. Thus, the H-R diagram not only shows us the colors and luminosities of many stars, it shows these stars at different stages in their evolutionary histories.

All stars on the main sequence have interiors hot enough fuse four hydrogen atoms into one helium atom, and this one helium atom is 0.7% lighter than 4 hydrogen atoms were. The lost mass is converted into energy, and this energy is released, providing the star's luminosity. Over billions of years, however, the residual helium in the star's core accumulates. When enough helium has accumulated, the helium can also undergo nuclear reaction. In this reaction, three helium atoms are converted into one carbon atom. The helium-burning nuclear reaction can occur only when the star's interior reaches a higher temperature, and this higher temperature causes the star's outer surface to expand to a much larger size than it was while it remained on the main sequence. Even though the core of the star is much hotter, the surface is now cooler, making the star redder. Thus, over time, a star becomes a red giant, moving from the main sequence area in the center of the H-R diagram to the red giant area in the upper right. The diagram below shows how a Sun-like star evolves on the H-R diagram.

 An H-R diagram showing the evolutionary track of a sun-like star*

The evolution from main sequence to red giant occurs at different times for different stars. Stars that are much heavier and hotter, like O-stars, become red giants in only 10 million years. Cooler, lighter stars like our sun take 10 billion years to become red giants. This fact actually provides a way of testing how old a group of stars is - jut make an H-R diagram for the stars, and see which classes of stars have evolved off the main sequence!

Eventually, all the helium in the core of the star is used up. At this point, what happens next depends on the mass of the star. The heaviest stars, over six to eight times as massive as our sun, have enough pressure in their cores to start fusing carbon. Once carbon is gone, they explode as supernovae, leaving behind neutron stars or a black holes. Less massive stars simply burn out, shedding their outer layers into beautiful planetary nebulae, and leaving the core as a hot white dwarf. White dwarfs lie in the lower left corner of the H-R diagram, a cosmic burial ground for dead stars.

## Astronomy > One-on-One with the Sun

STELLA: You know, everyone here on Earth loves your work. If it weren’t for you, we wouldn’t be here.

SUN: Thanks, Stella. I’ve been cranking out energy for billions of years—and not one day off.

STELLA: Impressive. Your agent told me that you’re the biggest star in the universe . Is that true?

SUN: Actually, no. My agent tends to exaggerate. I’m just a medium-sized star. To my fans on Earth, I look much bigger than the other stars in the sky. That’s because I’m much closer to you than the other stars. I’m 93 million miles away from you. The bigger stars are millions of times farther away from your planet.

STELLA: I can’t believe it. You, the Sun, are just an average, ordinary, run-of-the-mill star!

SUN: Hey, take it easy. I may be an average star, but I’m still a LOT bigger than your puny planet. Guess how many Earth-sized planets could fit inside me?

STELLA: I don’t know. Maybe ten?

SUN: Not even close! More than a million! I'm big and I've got lots of mass. That's why I've got a lot of gravity. You guys don’t call this the Solar system for nothing. "Solar" means "sun." This is my show. All the planets, comets, and asteroids orbit around ME.

STELLA: I know you star types tend to be touchy about age, but how old are you?

SUN: Well, I began shining about 4.6 billion years ago. It was so long ago I don’t remember the exact day.

STELLA: Was it a Sunday? Just kidding. Actually, I’m curious to know how stars begin. What’s your story?

SUN: We stars begin as huge spinning clouds of gas and dust. Gravity pulls the gas and dust together in round clumps, which get hot. At that point, we're called protostars. Eventually we get hot enough to start fusing hydrogen atoms together, which releases energy. Then we really shine. Stars are born in batches, which are like star nurseries. Then, over millions of years, these stars drift apart and spread out around the galaxy.

STELLA: Let’s turn to a delicate subject. How do stars die?

SUN: After stars like me shine for billions of years, we eventually run out of fuel. When that happens, we swell up as much as 100 times bigger and are called red giants. Astronomers predict that I’ll eventually become a red giant. At that time, my heat will vaporize the inner planets - Mercury , Venus , and Earth .

The most massive of supergiant stars are known as hypergiants. However, these stars have a very loose definition, they are usually just red (or sometimes blue) supergiant stars that are the highest order: the most massive and the largest.

A very high-mass star will oscillate between different supergiant stages as it fuses heavier and heavier elements in its core. Eventually, it will exhaust all its nuclear fuel that runs the star. When that happens, gravity wins. At that point, the core is primarily iron (which takes more energy to fuse than the star has) and the core can no longer sustain outward radiation pressure, and it begins to collapse.

The subsequent cascade of events leads, eventually to a Type II supernova event. Left behind will be the core of the star, having been compressed due to the immense gravitational pressure into a neutron star or in the cases of the most massive of stars, a black hole is created.

## Asymptotic Giant Branch Stars

This book deals with stars during a short episode before they undergo a ma­ jor, and fatal, transition. Soon the star will stop releasing nudear energy, it will become a planetary nebula for abrief but poetic moment, and then it will turn into a white dwarf and slowly fade out of sight. Just before this dramatic change begins the star has reached the highest luminosity and the largest diameter in its existence, and while it is a star detectable in galaxies beyond the Local Group, its structure contains already the inconspicuous white dwarf it will become. It is called an "asymptotic giant branch star" or "AGB star". Over the last 30 odd years AGB stars have become a topic of their own although individual members of this dass had already been studied for cen­ turies without realizing what they were. In the early evolution, so called "E-AGB"-phase, the stars are a bit bluer than, but otherwise very similar to, what are now called red giant branch stars (RGB stars). It is only in the sec­ ond half of their anyhow brief existence that AGB stars differ fundamentally from RGB stars.

"AGB stars have red giants as progenitors, and are fated to become planetary nebulae and white dwarfs. This book comprises a set of papers dealing with all aspects of the physics of these stars. It is intended for a graduate study level … . the chapters which are designed to be self-standing do provide interesting discussions about theory and corroborative observations, and I think there is lot that can be gleaned … ." (Callum Potter, Journal of the British Astronomical Association, Vol. 114 (3), 2004)

## Hubble Finds Three Giant Exoplanets and Several Brown Dwarfs in Orion Nebula

Using the NASA/ESA Hubble Space Telescope to peer deep into the famous Orion Nebula, Space Telescope Institute researcher Massimo Robberto and colleagues searched for small, faint bodies. They found 17 very-low-mass brown dwarf companions to red dwarf stars, one brown dwarf pair, and one brown dwarf with a planetary companion. The astronomers also found three giant exoplanets, including a binary system where two planets orbit each other in the absence of a host star.

This Hubble image shows the Orion Nebula. Image credit: NASA / ESA / M. Robberto, Space Telescope Science Institute & ESA / Hubble Space Telescope Orion Treasury Project Team.

The Orion Nebula, also known as NGC 1976, Messier 42 (M42), LBN 974, and Sharpless 281, is a diffuse nebula in the constellation Orion.

It spans about 24 light-years and is located approximately 1,350 light-years away from Earth. It can be seen with the naked eye as a fuzzy patch surrounding the star Theta Orionis in the Hunter’s Sword, below Orion’s belt.

The Orion Nebula is an excellent laboratory for studying the star formation process across a wide range, from opulent giant stars to diminutive red dwarf stars and elusive, faint brown dwarfs.

Because brown dwarfs are colder than stars, Dr. Robberto and co-authors used Hubble to identify them by the presence of water in their atmospheres.

“These are so cold that water vapor forms. Water is a signature of substellar objects. It’s an amazing and very clear mark. As the masses get smaller, the stars become redder and fainter, and you need to view them in the infrared. And in infrared light, the most prominent feature is water,” Dr. Robberto explained.

“But hot water vapor in the atmosphere of brown dwarfs cannot be easily seen from Earth’s surface, due to the absorbing effects of water vapor in our own atmosphere. Fortunately, Hubble is up above the atmosphere and has near-infrared vision that can easily spot water on distant worlds.”

The astronomers identified 1,200 candidate reddish stars. They found that the stars split into two distinct populations: those with water, and those without. The bright ones with water were confirmed to be faint red dwarfs.

This image is part of a Hubble survey for low-mass stars, brown dwarfs, and planets in the Orion Nebula. Each symbol identifies a pair of objects, which can be seen in the symbol’s center as a single dot of light. Special image processing techniques were used to separate the starlight into a pair of objects. The thicker inner circle represents the primary body, and the thinner outer circle indicates the companion. The circles are color-coded: red for a planet orange for a brown dwarf and yellow for a star. Located in the upper left corner is a planet-planet pair in the absence of a parent star. In the middle of the right side is a pair of brown dwarfs. The portion of the Orion Nebula measures roughly 3 x 4 light-years. Image credit: NASA / ESA / G. Strampelli, STScI.

Compass image for substellar objects in the Orion Nebula. Image credit: NASA / ESA / G. Strampelli, STScI.

The team also looked for fainter, binary companions to these 1,200 reddish stars.

Because they are so close to their primary stars, these companions are nearly impossible to discover using standard observing methods.

But by using a unique, high-contrast imaging technique developed by the astronomers, they were able to resolve faint images of a large number of candidate companions.

This first analysis did not allow Hubble astronomers to determine whether these objects orbit the brighter star or if their proximity in the Hubble image is a result of chance alignment.

As a consequence, they are classified as candidates for now. However, the presence of water in their atmospheres indicates that most of them cannot be misaligned stars in the galactic background, and thus must be brown dwarfs or exoplanet companions.

In all, the researchers found 17 candidate brown dwarf companions to red dwarf stars, one brown dwarf pair, and one brown dwarf with a planetary companion.

The study also identified three potential planetary mass companions: one associated to a red dwarf, one to a brown dwarf, and one to another planet.

The scientists presented their results this week at the 231st Meeting of the American Astronomical Society in Washington, D.C.

Giovanni Maria Strampelli et al. 2018. A HST/WFC3 Search for Substellar Companions in the Orion Nebula Cluster. 231st AAS Meeting, abstract # 414.07

## Will it be dangerous when close stars become Red Giants? - Astronomy

The term star was originally associated with the visible stars we recognise from the night sky. Stellar means “star-like”.

As our knowledge of the Universe increased, it was soon realised that our Sun was a fairly normal star, just close enough to be very bright due to the effect of the inverse square law.

The scientific use of the telescope brought many stars into view for the first time, and astronomers now believe there are some

Stars have a wide range of masses, and their luminosity varies by many orders of magnitude. As stars increase in mass their lifetimes become dramatically shorter, with stars 10 times that of the Sun living for only about 0.1% of the time, albeit at much greater luminosity (about 10,000 times brighter). Astronomers refer to how stars live and die as stellar evolution even though it has nothing to do with Darwin’s theories.

Stars like our Sun live for about 10 Billion years before they exhaust their primary source of fuel, the simplest element, hydrogen. After this occurs they swell up dramatically becoming red giants before losing their outer layers and resembling a planetary nebula. Once the outer layers peel off, the star becomes known as a white dwarf. White dwarfs are still referred to as stars. Whilst burning hydrogen in their cores, stars are said to be “on the main sequence” of the Hertzsprung Russell diagram.

Stars originally more massive than about 6-8 times the mass of the Sun can burn elements more massive than Hydrogen, and ultimately create cores that collapse catastrophically, creating neutron stars or black holes in a supernova explosion. Neutron stars and black holes are frequently referred to as stars, even though they are frequently invisible at optical wavelengths. The exact mass at which a star ceases to form a neutron star and starts creating a black hole is not known, but thought to be around 20 solar masses. Neutron stars manifest themselves in various ways, among them pulsars and magnetars.

Stars less massive than about 0.8 solar masses have not had sufficient time to exhaust their hydrogen since the Big Bang, and are still on the main sequence.

Very low-mass stars with masses less than about 0.08 solar masses, cannot burn Hydrogen at all in their cores and are often called “brown dwarfs”.

Stars are not formed individually, but in massive groups and are usually associated with galaxies (collections of billions of stars) or globular clusters.

Shooting stars have nothing to do with stars whatsoever, and are small particles striking the Earth’s atmosphere.

Study Astronomy Online at Swinburne University
All material is © Swinburne University of Technology except where indicated.

Over the past ten years, researchers studying rocks from the Earth and meteorites have been able to demonstrate these so-called isotopic anomalies for more and more elements. Schönbächler and her group have been looking at meteorites that were originally part of asteroid cores that were destroyed a long time ago, with a focus on the element palladium.

Other teams had already investigated neighbouring elements in the periodic table, such as molybdenum and ruthenium, so Schönbächler’s team could predict what their palladium results would show. But their laboratory measurements did not confirm the predictions. “The meteorites contained far smaller palladium anomalies than expected,” says Mattias Ek, postdoc at the University of Bristol who made the isotope measurements during his doctoral research at ETH.

Now the researchers have come up with a new model to explain these results, as they report in the journal Nature Astronomy. They argue that stardust consisted mainly of material that was produced in red giant stars. These are aging stars that expand because they have exhausted the fuel in their core. Our sun, too, will become a red giant four or five billion years from now.

In these stars heavy elements such as molybdenum and palladium were produced by what is known at the slow neutron capture process. “Palladium is slightly more volatile than the other elements measured. As a result, less of it condensed into dust around these stars, and therefore there is less palladium from stardust in the meteorites we studied” Ek says.

The ETH researchers also have a plausible explanation for another stardust puzzle: the higher abundance of material from red giants on Earth compared to Mars or Vesta or other asteroids further out in the solar system. This outer region saw an accumulation of material from supernova explosions.

“When the planets formed, temperatures closer to the Sun were very high,” Schönbächler explains. This caused unstable grains of dust, for instance those with an icy crust, to evaporate. The interstellar material contained more of this kind of dust that was destroyed close to the Sun, whereas stardust from red giants was less prone to destruction and hence concentrated there. It is conceivable that dust originating in supernova explosions also evaporates more easily, since it is somewhat smaller. “This allows us to explain why the Earth has the largest enrichment of stardust from red giant stars compared to other bodies in the solar system” Schönbächler says.

The author of this text, Barbara Vonarburg, is in charge of public outreach at the National Competence Center in Research PlanetS .

## Beyond Earthly Skies

6 billion years or so, the Sun will start running out of hydrogen in its core and being to enter its post-main-sequence phase of evolution chara cterised by a large increase in its luminosity. All the planets circling the Sun will receive much greater insolation than they do now. Presently, Jupiter orbits the Sun at a distance of roughly 5 AU, where 1 AU is the average Earth-Sun separation distance. When the Sun enters post-main-sequence evolution, Jupiter might become so intensely irradiated that it becomes a “hot-Jupiter”.

1000 K or more when the Sun goes through its RGB and AGB stages. Many of the currently known Jupiter-mass planets in wide, several-AU orbits around Sun-like stars (i.e. stars between 1 to 3 times the Sun’s mass) will also experience such a temperature increase when their host stars evolve off the main-sequence. The authors term such planets “red giant hot-Jupiters” (RGHJs) to distinguish them from typical hot-Jupiters that circle in short-period, close-in orbits around main-sequence stars.

1/10,000th of the planet’s mass for a Jupiter-mass planet.

300 K) for water-ice to sublimate. The abundance of H2O drops slights for a brief period during the RGB stage when atmospheric temperatures on the RGHJ exceed

600 K. At such temperatures, some of the oxygen in H2O becomes bounded in silicates. The same drop in H2O abundance might also occur when temperatures rise again during the AGB stage.