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If our sun was already in the process of becoming a red giant, would the gradual rise in luminosity be noticeable to our eyes at some point in human existence?
If our sun was already in the process of becoming a red giant, would the gradual rise in luminosity be noticeable to our eyes at some point in human existence?
I'd say no for a couple of reasons.
FWIW, in about 1.1 billion years it will be too hot for most lifeforms currently on Earth, long before the Sun starts moving off the main sequence and becoming a red giant. See Timeline of the far future.
Our eyes and vision processes accommodate (adjust for) changes in brightness, we walk indoors from a sunny day to a well lit room and the brightness of things has dropped by 99% or nearly a factor of 100 within a few seconds and yet though we are somewhat aware of the change of tens of percent per second we hardly notice it.
Our vision system is constantly adjusting its white point to accommodate changes in the color of available light.† A white sheet of paper looks white to us even as the color of ambient light changes because our vision is always "color balancing" (not exactly the right term but it's something like that).
†From @Luaan's comment
It's called "white balancing" - calibrating vision for ambient light to make white surfaces (hopefully) appear white. Sometimes it doesn't quite work properly, and you get fun things like The Dress (since different people's vision chooses different arbitrary ambient conditions, and end up disagreeing wildly about the colors involved)
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ( M ☉)) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K (4,700 °C 8,500 °F) or lower. The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars.
Red giants vary in the way by which they generate energy:
- most common red giants are stars on the red-giant branch (RGB) that are still fusing hydrogen into helium in a shell surrounding an inert helium core stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process (AGB) stars with a helium burning shell outside a degenerate carbon–oxygen core, and a hydrogen-burning shell just beyond that.
Many of the well-known bright stars are red giants, because they are luminous and moderately common. The K0 RGB star Arcturus is 36 light-years away, and Gamma Crucis is the nearest M-class giant at 88 light-years' distance.
Red giants evolve out of main-sequence stars that have masses in the range from around 0.3 solar masses to around 8 solar masses. Stars initially form from collapsing molecular clouds in the interstellar medium.
These clouds contain hydrogen and helium, with trace amounts of metals, and all of these elements are uniformly mixed throughout the star.
The star reaches the main sequence when the core reaches a temperature high enough to begin fusing hydrogen and establishes hydrostatic equilibrium.
Over a star’s main sequence life, it slowly converts hydrogen in the core to helium. A star’s main sequence life ends when nearly all its hydrogen supplies in the core have been fused.
When the hydrogen supplies are exhausted, nuclear reactions can no longer continue and thus the core begins to contract due to its own gravity.
This brings additional hydrogen into a zone where the temperature and pressure are sufficient to cause fusion to resume in a shell around the core.
The hydrogen-burning shell results in a situation that has been described as the mirror principle, when the core within the shell contracts, the layers of the star outside the shell must expand.
The evolutionary path the star takes as it moves along the red-giant phase depends solely on its mass. For example, the Sun and stars of less than 2 solar masses, the core will become dense enough that electron degeneracy pressure will prevent it from collapsing further.
Once the core is degenerate, it will continue to heat until it reaches a temperature of roughly 108 K, hot enough to begin fusing helium to carbon via the triple-alpha process.
When the degenerate core reaches this temperature, the entire core will begin helium fusion nearly simultaneously in a so-called helium flash.
More massive stars will have their collapsing cores reach 108 K before it is dense enough to be degenerate, thus helium fusion will begin much more smoothly and won’t produce a helium flash.
An analogous process occurs when the central helium is exhausted and the star collapses once again, causing helium in a shell to begin fusing.
At the same time, hydrogen may begin fusion in a shell just outside the burning helium shell, which puts the star onto the asymptotic giant branch, a second red-giant phase.
A star that has fewer than 8 solar masses will never start fusion in its degenerate core. However, at the end of its second phase, the star will eject its outer layers, forming a planetary nebula, and having its core exposed, ultimately becoming a white dwarf.
So, um, maybe the Sun *will* eventually swallow the Earth. Bummer.
The good news is that when the Sun starts to die and swells into a red giant we'll have great seats!
More Bad Astronomy
The bad news is that those seats will be inside the Sun.
If it helps, you can postpone your panic for about 7 billion years. That's when the show really starts.
I wrote about this a bit a few days ago, about what happens a star like the Sun starts to die. But to recap:
The Sun is currently a very stable star, with hydrogen in the core merrily undergoing nuclear fusion, transmogrifying itself into helium and energy. The energy leaking out of the core and into the upper layers is what keeps the Sun inflated, like a hot air balloon supporting itself. It's been doing this for over four billion years, and that's where we are now.
But, over time, helium builds up in the core. It takes a lot more pressure and higher temperature to fuse helium, so for now the helium is inert. It just sits there, slowly heating up. In roughly 6 billion years the Sun will run out of hydrogen in the core, but the core will shrink and heat up so much that hydrogen will fuse in a thin shell just outside the core. That dumps more helium into the core, contracting it and heating it further. Eventually, after some hundreds of millions of years further, conditions get so dire that helium itself fuses. The process above repeats, except this time helium fuses into carbon and oxygen, which build up in the core, generating vast amounts of energy * .
All this is happening deep in the Sun's core. The outer layers react to this, slowly, but react they do. When hydrogen shell fusion starts the outer layers will swell up enormously, turning the Sun into a red giant. It'll shrink somewhat when helium fusion begins, then swell up again when the carbon and oxygen build up.
The size of the Sun now (1.4 million km) compared to when it becomes a red giant in about 7 billion years. 1 AU is the distance from the Earth to the Sun now, 150 million km. Credit: Oona Räisänen
This is where things get fun, if lighting a blowtorch to the inner planets is your idea of fun.
When the Sun goes red giant, it'll get big enough to engulf Mercury and Venus for sure. They will literally exist for some time inside the Sun, orbits decaying as they plow through the hot plasma, falling farther in and eventually vaporizing entirely.
The Earth's fate isn't so clear. As the Sun expands, it starts to blow a wind of subatomic particles, like the solar wind now but much denser. The Sun will lose enough mass that its gravity will weaken, and that means the planets' orbits will expand. The problem is that the Earth is pretty much on the line dividing getting swallowed by the red giant Sun and moving away enough to escape that fate. It depends on the detailed physics, like how much mass loss the Sun will undergo, and I've seen the arguments in journals go back and forth for a while now.
Artwork depicting the Earth cooked by the Sun when it becomes a red giant… provided it doesn’t get engulfed as the Sun expands. Credit: Wikimedia commons / fsgregs
Well, I just so happened to be talking to my old Master's degree advisor, Noam Soker, about dying stars recently, and he casually mentioned that he's done the math! In a paper published in 2018 he looked into how much mass the Sun will lose. If a dying star engulfs a big planet (like Jupiter), then it will lose a lot of mass as the planet orbiting inside it spins it up like a fork whipping up a bowl of eggs. The star spins more rapidly, making it easier to throw off matter. Many stars do indeed have such big planets on tight orbits.
But if the star lacks such a close-in planet, it won't lose nearly as much mass. That means that as these stars expand, they'll still be holding on to their inner planets.
Yeah, that's us. Mercury and Venus won't do much inside the Sun but fizzle and die, so the Sun's wind won't be very strong, so it'll keep more mass, which means its gravity won't drop as much as previously thought.
Which means, in turn, the Earth may very well still be close in as the Sun expands to a red giant. And, well, that sucks for us. The Sun will eat our planet.
The Sun’s size over time (starting about 7.5 billion years from now Gyr = gigayears) in terms of its size now (top panel the Sun’s current radius is 700,000 km), and in terms of the size of Earth’s orbit (bottom). Different physical models are shown in different colors, but show the Sun expanding, contracting, and expanding again over time. In the bottom panel, if the Sun’s size gets over 1, the Earth will be inside the Sun. That happens in several models. Credit: Sobach and Soker
Not that the alternative was great, mind you. Even if the Earth managed to stay outside the Sun, the Sun will expand so much it will fill Earth's sky. It will be so hot that the solid rock of our planet will boil, so being inside or outside the Sun is kindof a technicality at that point. Either way, yikes. Cooked planet.
To remind you, we're talking about events that play out roughly 7–8 billion years from now. If you're worried about your 401k or the upcoming election, this isn't something that should weigh too heavily on you. And hey, if it helps, the Sun is slowly heating up now, and in a few hundred million years the Earth will be uninhabitable anyway!
I rather hope that whatever humanity looks like in that far-flung future, we're smart enough to have left for cooler climes out there in the galaxy. Who knows, maybe we'll even tow the Earth to a safe distance (which, to be honest, is difficult, as the Sun will fluctuate a lot during those red giant phases so the Earth will need to be moved around quite a bit).
The way I see things, this is of scientific interest, and even human interest, but there are far more pressing matters calling for our attention now. It's always interesting to wonder what the far future holds, and there may be important things to learn from it, but one lesson I take is that we need to appreciate the time we have now, and make it as best we can. Nothing lasts forever. Not even planets, or stars.
* The actual physics and process going on is more detailed than this, and way cooler, but you don't need to know it for what I'm discussing here. But if you want to know more, well, I know a guy.
Giant red stars may heat frozen worlds into habitable planets
An artist's conception of star scorching its nearby exoplanet. New research shows that aging red giant stars, far from destroying life, could warm frozen worlds into habitable homes. Credit: ESO/L. Calçada
Searching vast cosmic communities like real estate agents rifling through listings, Cornell astronomers now hunt through time and space for habitable exoplanets &ndash planets beyond our own solar system &ndash looking at planets flourishing in old star, red giant neighborhoods.
Astronomers search for these promising worlds by looking for the &ldquohabitable zone,&rdquo the region around a star in which water on a planet&rsquos surface is liquid and signs of life can be remotely detected by telescopes.
&ldquoWhen a star ages and brightens, the habitable zone moves outward and you&rsquore basically giving a second wind to a planetary system,&rdquo said Ramses M. Ramirez, research associate at Cornell&rsquos Carl Sagan Institute and lead author of the study. &ldquoCurrently objects in these outer regions are frozen in our own solar system, like Europa and Enceladus &ndash moons orbiting Jupiter and Saturn.&rdquo
In their work, Ramirez and Lisa Kaltenegger, associate professor of astronomy and director of the Sagan Institute, have modeled the locations of the habitable zones for aging stars and how long planets can stay in it. Their research, &ldquoHabitable Zones of Post-Main Sequence Stars,&rdquo was published May 16 in the Astrophysical Journal.
All throughout the universe there are stars in varying phases and ages. The oldest detected Kepler planets (exoplanets found using NASA&rsquos Kepler telescope) are about 11 billion years old, and the exoplanetary diversity suggests that around other stars, such initially frozen worlds could be the size of Earth and could provide habitable conditions once the star becomes older. Astronomers usually looked at middle-aged stars like our sun, but to find habitable worlds, one needs to look around stars of all ages, Kaltenegger said.
Dependent upon the mass of the original star, planets and their moons loiter in this red giant habitable zone up to 9 billion years. Earth, for example, has been in our sun&rsquos habitable zone so far for about 4.5 billion years, and it has teemed with changing iterations of life. However, in a few billion years our sun will become a red giant, engulfing Mercury and Venus, turning Earth and Mars into sizzling rocky planets, and warming distant worlds like Jupiter, Saturn and Neptune &ndash and their moons &ndash in a newly established red giant habitable zone.
&ldquoFor stars that are like our sun, but older, such thawed planets could stay warm up to half a billion years. That&rsquos no small amount of time,&rdquo said Ramirez.
Said Kaltenegger: &ldquoIn the far future, such worlds could become habitable around small red suns for billions of years, maybe even starting life, just like Earth. That makes me very optimistic for the chances for life in the long run.&rdquo
This research was supported by the Simons Foundation and by the Carl Sagan Institute.
22.1 Evolution from the Main Sequence to Red Giants
One of the best ways to get a “snapshot” of a group of stars is by plotting their properties on an H–R diagram . We have already used the H–R diagram to follow the evolution of protostars up to the time they reach the main sequence. Now we’ll see what happens next.
Once a star has reached the main-sequence stage of its life, it derives its energy almost entirely from the conversion of hydrogen to helium via the process of nuclear fusion in its core (see The Sun: A Nuclear Powerhouse). Since hydrogen is the most abundant element in stars, this process can maintain the star’s equilibrium for a long time. Thus, all stars remain on the main sequence for most of their lives. Some astronomers like to call the main-sequence phase the star’s “prolonged adolescence” or “adulthood” (continuing our analogy to the stages in a human life).
The left-hand edge of the main-sequence band in the H–R diagram is called the zero-age main sequence (see Figure 18.15). We use the term zero-age to mark the time when a star stops contracting, settles onto the main sequence, and begins to fuse hydrogen in its core. The zero-age main sequence is a continuous line in the H–R diagram that shows where stars of different masses but similar chemical composition can be found when they begin to fuse hydrogen.
Since only 0.7% of the hydrogen used in fusion reactions is converted into energy, fusion does not change the total mass of the star appreciably during this long period. It does, however, change the chemical composition in its central regions where nuclear reactions occur: hydrogen is gradually depleted, and helium accumulates. This change of composition changes the luminosity, temperature, size, and interior structure of the star. When a star’s luminosity and temperature begin to change, the point that represents the star on the H–R diagram moves away from the zero-age main sequence.
Calculations show that the temperature and density in the inner region slowly increase as helium accumulates in the center of a star. As the temperature gets hotter, each proton acquires more energy of motion on average this means it is more likely to interact with other protons, and as a result, the rate of fusion also increases. For the proton-proton cycle described in The Sun: A Nuclear Powerhouse, the rate of fusion goes up roughly as the temperature to the fourth power.
If the rate of fusion goes up, the rate at which energy is being generated also increases, and the luminosity of the star gradually rises. Initially, however, these changes are small, and stars remain within the main-sequence band on the H–R diagram for most of their lifetimes.
Star Temperature and Rate of Fusion
Check Your Learning
The temperature would increase by a factor of 256 0.25 (that is, the 4 th root of 256), or 4 times.
Lifetimes on the Main Sequence
How many years a star remains in the main-sequence band depends on its mass. You might think that a more massive star, having more fuel, would last longer, but it’s not that simple. The lifetime of a star in a particular stage of evolution depends on how much nuclear fuel it has and on how quickly it uses up that fuel. (In the same way, how long people can keep spending money depends not only on how much money they have but also on how quickly they spend it. This is why many lottery winners who go on spending sprees quickly wind up poor again.) In the case of stars, more massive ones use up their fuel much more quickly than stars of low mass.
The reason massive stars are such spendthrifts is that, as we saw above, the rate of fusion depends very strongly on the star’s core temperature. And what determines how hot a star’s central regions get? It is the mass of the star—the weight of the overlying layers determines how high the pressure in the core must be: higher mass requires higher pressure to balance it. Higher pressure, in turn, is produced by higher temperature. The higher the temperature in the central regions, the faster the star races through its storehouse of central hydrogen. Although massive stars have more fuel, they burn it so prodigiously that their lifetimes are much shorter than those of their low-mass counterparts. You can also understand now why the most massive main-sequence stars are also the most luminous. Like new rock stars with their first platinum album, they spend their resources at an astounding rate.
The main-sequence lifetimes of stars of different masses are listed in Table 22.1. This table shows that the most massive stars spend only a few million years on the main sequence. A star of 1 solar mass remains there for roughly 10 billion years, while a star of about 0.4 solar mass has a main-sequence lifetime of some 200 billion years, which is longer than the current age of the universe. (Bear in mind, however, that every star spends most of its total lifetime on the main sequence . Stars devote an average of 90% of their lives to peacefully fusing hydrogen into helium.)
|Spectral Type||Surface Temperature (K)||Mass |
(Mass of Sun = 1)
|Lifetime on Main Sequence (years)|
These results are not merely of academic interest. Human beings developed on a planet around a G-type star. This means that the Sun’s stable main-sequence lifetime is so long that it afforded life on Earth plenty of time to evolve. When searching for intelligent life like our own on planets around other stars, it would be a pretty big waste of time to search around O- or B-type stars. These stars remain stable for such a short time that the development of creatures complicated enough to take astronomy courses is very unlikely.
From Main-Sequence Star to Red Giant
Eventually, all the hydrogen in a star’s core, where it is hot enough for fusion reactions, is used up. The core then contains only helium, “contaminated” by whatever small percentage of heavier elements the star had to begin with. The helium in the core can be thought of as the accumulated “ash” from the nuclear “burning” of hydrogen during the main-sequence stage.
Energy can no longer be generated by hydrogen fusion in the stellar core because the hydrogen is all gone and, as we will see, the fusion of helium requires much higher temperatures. Since the central temperature is not yet high enough to fuse helium, there is no nuclear energy source to supply heat to the central region of the star. The long period of stability now ends, gravity again takes over, and the core begins to contract. Once more, the star’s energy is partially supplied by gravitational energy, in the way described by Kelvin and Helmholtz (see Sources of Sunshine: Thermal and Gravitational Energy). As the star’s core shrinks, the energy of the inward-falling material is converted to heat.
The heat generated in this way, like all heat, flows outward to where it is a bit cooler. In the process, the heat raises the temperature of a layer of hydrogen that spent the whole long main-sequence time just outside the core. Like an understudy waiting in the wings of a hit Broadway show for a chance at fame and glory, this hydrogen was almost (but not quite) hot enough to undergo fusion and take part in the main action that sustains the star. Now, the additional heat produced by the shrinking core puts this hydrogen “over the limit,” and a shell of hydrogen nuclei just outside the core becomes hot enough for hydrogen fusion to begin.
New energy produced by fusion of this hydrogen now pours outward from this shell and begins to heat up layers of the star farther out, causing them to expand. Meanwhile, the helium core continues to contract, producing more heat right around it. This leads to more fusion in the shell of fresh hydrogen outside the core (Figure 22.2). The additional fusion produces still more energy, which also flows out into the upper layer of the star.
Most stars actually generate more energy each second when they are fusing hydrogen in the shell surrounding the helium core than they did when hydrogen fusion was confined to the central part of the star thus, they increase in luminosity. With all the new energy pouring outward, the outer layers of the star begin to expand, and the star eventually grows and grows until it reaches enormous proportions (Figure 22.3).
When you take the lid off a pot of boiling water, the steam can expand and it cools down. In the same way, the expansion of a star’s outer layers causes the temperature at the surface to decrease. As it cools, the star’s overall color becomes redder. (We saw in Radiation and Spectra that a red color corresponds to cooler temperature.)
So the star becomes simultaneously more luminous and cooler. On the H–R diagram, the star therefore leaves the main-sequence band and moves upward (brighter) and to the right (cooler surface temperature). Over time, massive stars become red supergiants, and lower-mass stars like the Sun become red giants. (We first discussed such giant stars in The Stars: A Celestial Census here we see how such “swollen” stars originate.) You might also say that these stars have “split personalities”: their cores are contracting while their outer layers are expanding. (Note that red giant stars do not actually look deep red their colors are more like orange or orange-red.)
Just how different are these red giants and supergiants from a main-sequence star? Table 22.2 compares the Sun with the red supergiant Betelgeuse , which is visible above Orion’s belt as the bright red star that marks the hunter’s armpit. Relative to the Sun, this supergiant has a much larger radius, a much lower average density, a cooler surface, and a much hotter core.
|Mass (2 × 10 33 g)||1||16|
|Surface temperature (K)||5,800||3,600|
|Core temperature (K)||15,000,000||160,000,000|
|Luminosity (4 × 10 26 W)||1||46,000|
|Average density (g/cm 3 )||1.4||1.3 × 10 –7|
|Age (millions of years)||4,500||10|
Red giants can become so large that if we were to replace the Sun with one of them, its outer atmosphere would extend to the orbit of Mars or even beyond (Figure 22.4). This is the next stage in the life of a star as it moves (to continue our analogy to human lives) from its long period of “youth” and “adulthood” to “old age.” (After all, many human beings today also see their outer layers expand a bit as they get older.) By considering the relative ages of the Sun and Betelgeuse, we can also see that the idea that “bigger stars die faster” is indeed true here. Betelgeuse is a mere 10 million years old, which is relatively young compared with our Sun’s 4.5 billion years, but it is already nearing its death throes as a red supergiant.
Models for Evolution to the Giant Stage
As we discussed earlier, astronomers can construct computer models of stars with different masses and compositions to see how stars change throughout their lives. Figure 22.5, which is based on theoretical calculations by University of Illinois astronomer Icko Iben, shows an H–R diagram with several tracks of evolution from the main sequence to the giant stage. Tracks are shown for stars with different masses (from 0.5 to 15 times the mass of our Sun) and with chemical compositions similar to that of the Sun. The red line is the initial or zero-age main sequence. The numbers along the tracks indicate the time, in years, required for each star to reach those points in their evolution after leaving the main sequence. Once again, you can see that the more massive a star is, the more quickly it goes through each stage in its life.
Note that the most massive star in this diagram has a mass similar to that of Betelgeuse , and so its evolutionary track shows approximately the history of Betelgeuse. The track for a 1-solar-mass star shows that the Sun is still in the main-sequence phase of evolution, since it is only about 4.5 billion years old. It will be billions of years before the Sun begins its own “climb” away from the main sequence—the expansion of its outer layers that will make it a red giant.
Link to Learning
Use the Star in a Box simulation to explore the evolution of stars of different masses. Select the star’s mass, and then hit play to see how its luminosity, temperature, and size change during its lifetime.
In the life cycle of a star, how does a red giant become a planetary nebula?
Basically a Red Giant is formed when a Star like our Sun burns all of it's hydrogen to helium and then rearranges itself. This process takes about 10 Billion years. After becoming a Red Giant the Sun will become bigger and more denser than it is today.
At this time it will start burning Helium to Carbon for a few hundred million years until it runs out of Helium and since it will not be dense enough to form other heavier elements like Iron, the fusion process will stop, making the Star collapse on it's core due to inward acting gravity as there will be no Fusion energy to Stabilize this gravity.
At this time the Sun will calmly shed it's outer layers into Space called a Planetary nebula and become a White Dwarf, a cool extremely Dense Star, about the size of the earth but mass of the Sun.
Orbiting a Red Giant
A planet orbiting a giant red star has been discovered by an astronomy team led by Penn State’s Alex Wolszczan, who in 1992 discovered the first planets ever found outside our solar system. The new discovery is helping astronomers to understand what will happen to the planets in our solar system when our Sun becomes a red-giant star, expanding so much that its surface will reach as far as Earth’s orbit. Far in the future, the expanding sun will have drastic implications for the future of life on Earth. The star is 2 times more massive and 10 times larger than the Sun. The new planet circles the giant star every 360 days and is located about 300 light years from Earth, in the constellation Perseus. A paper describing the discovery will be published in a November 2007 issue of the Astrophysical Journal.
The discovery resulted from an ongoing effort that the research team began three years ago to find Jupiter-mass planets around red-giant stars that are typically farther from Earth than those included in most other planet searches. "After astronomers have spent more than 10 years searching for planets around Sun-like stars and discovering over 250 planets elsewhere in our galactic neighborhood, we still do not know whether our solar system’s properties, including life-supporting conditions on our planet, are typical or exceptional among solar systems throughout the Galaxy," Wolszczan says. "The picture for now, based on the searches for planets around stars like our Sun, is that our planetary system appears to be unusual in a number of ways."
"This planet is the first one discovered by Penn State astronomers with the Hobby-Eberly Telescope, and it is in one of the most distant of the ten published solar systems discovered around red-giant stars," comments Lawrence Ramsey, a member of the discovery team and the head of the Department of Astronomy and Astrophysics at Penn State. Ramsey is a leader in the conception, design, construction, and operation of the Hobby-Eberly Telescope. "We are now becoming serious participants in planetary searches and planetary astronomy using the Hobby-Eberly Telescope," he says.
Astronomers now are branching out with different strategies for searching for planets, with the hope of more quickly detecting life elsewhere in the universe, of discovering all the possible kinds of solar systems, and of learning how they form around different kinds of stars. Wolszczan’s team used one of these new strategies — searching for planets around giant stars, which have evolved to a later stage of life than our Sun’s.
"We have compiled a catalog of nearly a thousand giant stars that are candidates for hosting solar systems," Wolszczan says. Because the method for discovering planets involves repeated measurements of their gravitational effect on the star they circle, and because planets around red giants can take years to make one orbit around the star, the research team is just now beginning to reap discoveries from years of systematic observations. "It took us 3 years to gather enough data on over 300 stars to start identifying those that are good candidates for having planetary companions," Wolszczan said. "This planet is just the first of a number of planet discoveries that this research program is likely to produce."
This research is a collaboration between astronomers at Penn State, Nicholas Copernicus University in Poland, the McDonald Observatory, and the California Institute of Technology. "One important aspect of this work is that it marks the debut of a research group in Poland, led by Dr. Andrzej Niedzielski, which has become a serious contributor to discoveries in extra-solar planetary astronomy," Wolszczan said.
One reason for studying solar systems that include red-giant stars is that they help astronomers to understand more about the future of our own solar system — as family photos can give children an idea of what they might look like when they are the age of their grandparents. "Our Sun probably will make the Earth unhabitable in about 2 billion years because it will get hotter and hotter as it evolves on its way to becoming a red giant about 5 billion years from now," Wolszczan says. As the star swells up, transforming itself into a red giant, it affects the orbits of its planets and the dynamics of the whole planetary system, causing such changes as orbit crossings, planet collisions, and the formation of new planets out of the debris of those collisions. "When our Sun becomes a red giant, Earth and the other inner planets very likely will dive into it and disappear," Wolszczan says.
Another motivation for studying red-giant stars is to understand how their habitable zones move farther out as the star’s radiating surface becomes bigger. Based on how long it took for life to develop on Earth, scientists speculate that there is more than enough time during a star’s giant phase for life to get a start somewhere in the evolving habitable zones. "In our solar system, places like Europa &ndash a satellite of Jupiter that now is covered by a thick layer of water ice — might warm up enough to support life for more than a billion years or so, over the time when our Sun begins to evolve into a red giant, making life on Earth impossible," Wolszczan said.
The method the astronomers use to discover planets is to observe candidate stars, repeatedly measuring their space velocity using the Doppler effect — the changes in the star’s light spectrum that result from its being pulled alternately toward and away from Earth by the gravity of an orbiting planet. "When we detect a significant difference in a star’s velocity over a month or two, we then start observing that star more frequently," Wolszczan says. "In this paper, the velocity of the star changed by about 50 meters per second (about 100 miles per hour) between our first and second observations, so we observed that star more frequently and we found a clearly repeatable effect, indicating the presence of a planet." A star and its orbiting planet move around the center-of-mass of the whole system, so the star alternately approaches and recedes from Earth periodically. "When the star gets closer to us, its light becomes a little bit bluer and when it recedes from us, its light becomes redder, and we can measure that effect to deduce the presence of planets," Wolszczan explains.
Searching for planets around giant stars also is a clever way to learn about the formation of planets around stars more massive than our Sun. Because massive stars are so hot when they are in the phase of life of our Sun, astronomers have not been able to detect enough of their spectral lines to use the Doppler-spectroscopy method of finding planets. However, these stars become cooler as they evolve into giants, at which point the spectral-line observations needed for Doppler detection of planets become possible. "We want to know how often do planets form around stars that were more massive than our Sun," Wolszczan said. "Obviously, the more solar systems around red giants we discover and study, the better chance we have to really understand the big picture of planet formation."
Another reason astronomers are trying to discover planets around different kinds of stars at different stages of stellar evolution is to find out how different kinds of planetary systems change when their stars become red giants and how they ultimately end their lives as burnt-out, shrunken white-dwarfs.
"We really are at the very beginning of this effort and it is going to take time to get a consistent picture of planetary formation and evolution," Wolszczan says. "The more we learn, the greater the chance will be that sooner or later we will discover how ordinary or extraordinary is our home — the Earth’s solar system."
This research received financial support from NASA’s Jet Propulsion Laboratory, Penn State’s NASA-funded Astrobiology Program, the Polish Ministry of Science and Higher Education, and private donors.