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We often receive general questions about astronomy. Many people have the same questions, so we'll post the answers here, in this General Astronomy FAQ repository. When there are questions about a particular topic or project, we'll post them in the Projects section. Thanks for your interest!

General Astronomy Projects

General Astronomy Questions
by Shane Larson
Spring 2016

Shane is a Research Associate Professor in CIERA and also an astronomer in the Department of Astronomy at the Adler Planetarium. He is an expert science communicator who loves to share his knowledge of astronomy through outreach activities.

Please click to view the answers.

How long does it take to produce a star?

Stars are born out of vast interstellar clouds that we call "stellar nurseries." Regions of these clouds will sometimes have more gas than other regions, and the gravity will be a little bit stronger. That stronger gravity will draw gas in towards it, making the gravity even stronger, which will draw in even more gas. Over time, all the gas that becomes the star is drawn in. How quickly that happens depends on how much gas is in the cloud, and how big the star that is forming ends up being. A star like the Sun could take 50 million years or so to form, where as a much larger star than the sun would form in only a few million years.

But there is another interesting part of your question, which is how long did it take to make the stuff that forms a star? Our Sun is a descendant of much older stars in the Universe. Some previous generations of stars burned and lived out their lives, then exploded and threw all their gas back out into the galaxy, where it eventually collected and became the generation of stars that the Sun was born with. That process takes billions of years.

Where in the galaxy are most stars born?

If you look at a spiral galaxy, like the Milky Way, most of the star formation happens in the "disk" -- the large, flat part of the galaxy made up of the spiral arms. This is where most of the gas and molecular clouds in the galaxy are concentrated, and as the galaxy rotates that gas is moved around until enough gravity is concentrated to make the collapse we described above happen. Thinking on smaller scales, we might say that stars form in "stellar nurseries" -- dense clouds of gas -- that are located throughout the disk. We see many such examples in the Milky Way. Famous examples include the Orion Nebula (M42) and the Lagoon Nebula (M8). Usually many stars are born together out of a stellar nursery, forming a young cluster of stars called an "open cluster." In their early phases these clusters are more or less together, but there is not enough mutual attraction between the stars to keep them together for their entire lives, and the stars eventually drift apart and become single stars in the galaxy. Famous examples of open clusters include the Pleiades (M45), and the Butterfly Cluster (M6).

The Sun formed about 5 billion years ago in some open cluster that has long since dispersed. Something astronomers would very much like to do is to discover some of the stars in the Milky Way that are siblings of the Sun. Just being able to find one might tell us a lot about the early life of the Sun.

Are there certain gases needed in order for a star to be produced? If so, what gases?

Far and away the most dominant material in the entire Cosmos is hydrogen. That's good because the energy source in stars is nuclear fusion, and the easiest atom to use in nuclear fusion is hydrogen (4 hydrogens are put together to make helium -- this is source of energy in stars).

Stars have plenty of other kinds of atoms in them -- helium mostly, but also more familiar atoms like oxygen, carbon, nitrogen, and calcium are also there in tiny amounts. Helium is made from the nuclear fusion process, but the other atoms are in stars like the Sun because they came from the previous generation of stars that exploded and mad the gases that the Sun is made of. Stars are responsible for making all the elements on the periodic table. Everything up to iron can be made during nuclear fusion (depending on how big the star is), and everything after iron is made in supernova explosions.

What are black holes?

The simple definition of a black hole is: an object whose gravity is so strong you have to travel faster than the speed of light to get away from it.

This simple idea was first put forward as a mathematical idea in physics in 1783 by the Rev. John Michell. In modern astrophysics there is an additional bit to the definition: a black hole is an object whose gravity is so strong that if you get too close you will NEVER get away because you cannot travel faster than the speed of light (the ultimate speed limit in the Universe).

One of the interesting parts of science is that there are all kinds of things we can think about in the context of math, but that doesn't mean they have to happen in Nature. For a long time, we didn't know if black holes could actually exist or not. But for the last several decades, astronomers have seen many different phenomena in the Universe that can only be explained as being black holes, and we have come to accept them as part of modern astronomy. Most recently, you may have heard about the gravitational wave detection by LIGO announced in February -- this was the first detection every made of black holes themselves by measuring their gravitational signature here on Earth.

There are two kinds of black holes. Stellar mass black holes are a few to tens of times the mass of the Sun, and are formed from the supernova explosion of very big stars. Supermassive black holes are millions to billions of times the mass of the Sun, and are typically found in the centers of galaxies. The Milky Way has a 4 million solar mass black hole at its center (called "Sgr A*"); one of the most massive black holes ever seen was recently discovered in a galaxy called NGC 1600, weighing in at 17 billion times the mass of the Sun!

Why are some stars brighter than others?

There are a variety of reasons. First, let's suppose you have two exactly identical stars. They are both the same brightness, but if you put one much father away from the Earth than the other, it will appear dimmer. This is called the "apparent brightness" -- what you see from Earth, and it depends on the distance.

There are also physical reasons why different stars are intrinsically brighter. Hotter stars are usually brighter than dimmer stars, and bigger stars are brighter than smaller stars.

As astronomers, we are usually careful about describing the brightness of stars. When we are describing the intrinsic brightness, we imagine all the stars are exactly the same distance away from Earth and write down how bright they would be. The apparent brightness is the brightness we directly measure with our telescopes.

Do stars have color?

Absolutely! To your naked eye, many stars will have a distinct color to them -- Sirius and Vega are almost bluish, whereas Antares and Arcturus are very red. Color is one of the most important things we can measure about the stars, but we didn't always know what it meant. The first person to realize color was important was an astronomer named Annie Jump Cannon. She organized the stars into the "spectral classification scheme" that astronomers still use today. The reason color is important is it is related to the temperature of the star -- if you can measure the color, that is a direct measurement of the temperature. Blue stars are hotter, yellow stars are in the middle, and red stars are coolest. The connection between temperature and color was discovered by another astronomer, Cecilia Payne-Gaposchkin.

How big can supergiant star get?

The exact biggest size of a supergiant star is limited by many different aspects of the physics of stars. Some of the reasons include whether or not the energy made by fusion in the core can balance out the pull of gravity, and other reasons have to do with the amount of non-hydrogen gases in the star. The largest stars we expect to see are about 1500 times larger than the Sun, and there are many such stars we have seen in the Milky Way. There are several supergiant stars that are visible to your naked eye -- the most famous are Betelgeuse in the shoulder of Orion (about 1000x bigger than the Sun), Antares in the heart of Scorpius (about 850x bigger than the Sun), and Mu Cephei (about 1200x larger than the Sun) which is also called "Herschel's Garnet Star".

Do you believe there is life beyond our solar system?

Yes! This is an ongoing matter of discussion among astronomers, but I definitely fall on the optimistic side of the argument. My reasons are simple -- (1) the molecules that life is made of is plentiful and common in the Universe. (2) the chemistry of the most basic of lifeforms is not tremendously complicated. (3) we find life EVERYWHERE on Earth, even in places we don't expect life to easily survive!

What I'm imagining here is *any* kind of life, not necessarily intelligent life. Something as simple as algae would count as life, and thus is the reason for my optimism. Whether or not there is intelligent life, and more to the point whether we could communicate with other intelligent life -- that is a much harder question to be confident about!

In either case, this is a hard question for us to think about because we only know of one example of life in the Universe -- the life we see here on Earth! That means how we think about life and what we mean by life is limited by our experiences here on Earth. There is no reason to expect that life elsewhere would be the same as or even recognizable by the same standards that we use to talk about life here. Astronomers spend enormous amounts of time debating the question "what is life?" The answer to that question defines how we look for life and what we might think if and when we do find life elsewhere.

Is there other intelligent life in the universe?

One answer to this question is "yes" -- there are many intelligent lifeforms here on Earth other than humans! However, as we have discovered, it is notoriously difficult to define what we mean by "intelligence", and it is even more difficult to facilitate communication between the species on Earth. Never-the-less we see complex languages (e.g. in dolphins and other marine mammals), we see tool use (e.g. in birds and chimpanzees), and we see complex societal structures in many different species.

As for intelligent life not on Earth, the answer is "we don't know," but we are trying to figure that out. Our understanding of if there is life elsewhere in the Cosmos, and in particular if there is intelligent life elsewhere in the Cosmos, is profoundly limited by the fact that the ONLY example of life we know of is right here on Earth. We don't know how easy it is for life to arise, nor how easy it is for life to evolve into intelligent forms. Astronomers often frame this discussion in the context of something called "The Drake Equation," which is our attempt to estimate not just how many intelligent species there are out there, but how many there might be that are capable of sending and receiving radio messages.

Does Mars have life living on it? Why?

So far as we know, there is no life on Mars. Certainly there is no form of "large" life like we see on Earth -- no plants, no animals. We have robotic explorers on the surface, and many satellites in orbit, and in the many decades they have been exploring the Red Planet, we have seen no indication of large lifeforms. There maybe microbial life of some sort, but we have yet to find any evidence of it; the search is ongoing. One thing that is almost certainly true is that our spacecraft have likely some Earth microbes on them, though it is unclear how long they might survive on Mars.

Scientists do not understand the origin of life. We know that organic molecules (the building blocks of life) are plentiful throughout the Universe. We know that very soon after the formation of the Earth, life appeared, but for billions of years was really no more complicated than algae. We know Mars was once warmer and wetter, much like the Earth, so it might have been more "habitable" than it is now. So we ask the very interesting question: did Mars once harbor microbial life, just like the early Earth? Has that life survived somehow, or did it all perish as Mars' low gravity slowly lost its atmosphere and water to space? Can we find Martian life, hiding in the soil, or in underground water deposits? Could we find fossil evidence for life on Mars from its distant past?

Another interesting possibility is "panspermia." We know that both Earth and Mars were probably similar and habitable in their early phases. We also know that when asteroids hit planets, pieces of those planets get ejected into space as meteors that can and do find their way to other worlds. Is it possible that early life on Earth hitched a ride to Mars and still survives there today? Is it possible that life started on Mars, but then hitched a ride to Earth, where it flourished even as it died out on Mars? These are interesting questions, and are well within the realm of possibility, but we just don't know enough yet to give them definitive answers.

Could Mars have life on it in the future, and how do we know?

I think the answer to this is yes -- either because life is already there and we don't know it, or because we put life there (already, with our spaceprobes, or in the future with colonies). The reason is by analogy with Earth -- everywhere we look on Earth, we see life, no matter how extreme or inhospitable the environment. Life is very good at adapting to extreme environments -- it is easier for microbes, which cycle through generations and mutations far more rapidly than bigger life like us (you may have experienced this in biology lab, if you've ever grown bacterial colonies on anti-biotic plates, and discovered after a time a resistant branch of the colony that grows despite the anti-biotic). We often call dramatically adapted life "extremophiles." Classic examples on Earth include the algal mats around hot springs (like you see around thermal features in Yellowstone), worms and bacterial colonies at the bottom of the ocean around volcanic vents known as "black smokers", and bacteria that live in the cold, dry valleys of Antarctica. There are many other examples of microbes adapted to particular chemistry or adapted to symbiotic living with other species (think "lichen").

If you are asking about biogenesis -- the spontaneous emergence of life -- I think the answer is no, but only because humans will be there soon and disrupting whatever Nature might do if left to its own devices.

How does this affect our thinking about Mars and other planets?

Astrobiology is a very young discipline; we are just learning what the right questions to ask are, and how to go about looking for life. We are profoundly limited by our inexperience with the Universe, and profoundly biased by the fact that the only life we know of in on Earth.

On the one hand, we are just now learning about what planets can be like, and how many there are in the Universe; before about 20 years ago, we knew of no other worlds besides the planets around the Sun. Now we know of thousands, and are trying to understand what makes a planet "habitable" -- we are biased to think it should be like, he Earth, because that is the only experience with life we have. We think of habitability in terms of rocky surfaces, reasonable temperatures, and liquid water, because that is what we see on Earth. But if we were to ever discover balloon creatures drifting in the clouds of a gas giant like Jupiter, or crystalline life feeding off captured cosmic rays on the surface of a world like Pluto, we'd have to change our thinking -- that's okay, because that is the purpose of science.

On the other hand, we don't even know what questions to ask to decide if something is "alive." This is hard even on Earth! When we first sent Viking to Mars, there were several "life experiments" on board. Before we even launched, scientists agreed on what the outcomes of the experiments would mean, based on what they decided "life" would be like. When Viking sifted the sands of Mars, 3 of the 4 experiments gave positive signs of life! In the end, we changed our minds about what the outcome of the experiments meant. Why? Because we had not thought of every possibility; unexpectedly (then) the surface chemistry of Mars is dramatically different than expected because of the high levels of ultraviolet light on Mars (no ozone layer, like we have on Earth!). It's a hard field, and takes lots of people working on trying to define exactly what we mean by "life"!

What is the eventual fate of the Sun?

Stars, like people, are born, live their lives, and eventually perish. The difference between the lives of stars and the lives of people, however, is that the stars live far longer than any of us; the total length of time they live depends on their mass. Heavy, massive stars live short lives (in a cosmic sense) and small, less-massive stars live far longer lives. The Sun is an average sort of star. Stars of its size have about a 10 billion year lifespan, and it is currently about halfway through that time -- it is a "middled aged star." When it reaches the end of its life, it will enter a phase known as the "red giant phase" where it swells up to enormous size, expanding until it consumes most of the planets in the inner solar system; astronomers don't know exactly how large it will become, but it likely will expand out to the size of the Earth's orbit.

When the red giant phase is over, the Sun will shrug off a large fraction of its atmosphere, creating something called a "planetary nebula" -- a nearly spherical bubble of gas that expands outward, illuminated for perhaps 10,000 years by the shrinking ember of what was once the Sun (famous examples of planetary nebulae include the Ring Nebula in Lyra [M57] and the Cat's Eye Nebula [NGC 6543]). The left over remnant will be a small object a little bit less massive than the Sun, but about the size of the Earth called a "white dwarf." It will slowly fade and cool over billions of years until it becomes what astronomers call a "black dwarf" -- the last skeletal remains of the Sun.

What will happen to the Earth when the Sun dies?

All life on Earth depends on the Sun. First and foremost, the energy from the Sun keeps the Earth temperature in a comfortable range where water can exist in liquid state. If the Sun were not there, the Earth would be a frozen iceball.

Second, all life depends on the Sun for energy -- plants are at the base of the energy chain, harvesting sunlight and using that energy in photosynthesis to create carbohydrates. Animals eat the planets, and use the energy stored in the carbohydrates for their own energy needs. We eat the animals and the plants to get our energy.

But it is a fact that the Sun will someday die, and when it does, the Earth will die too. Astronomers expect sometime about a billion years before the red giant phase, the energy output of the Sun may change dramatically, boiling away all the water on Earth. When that happens, our planet will become inhospitable to life as we currently know it. Eventually, when the Sun becomes red giant, it will expand well out into the solar system and will burn the Earth into a cinder.

Will life survive? Ultimately the answer seems to be "no." The caveat to that depends ultimately on whether or not we develop the technology to leave the Earth and fly to other stars and other planets. There is certainly nothing that precludes that possibility, but we would never be able to take the entire ecosystem of the Earth with us --- trillions of species will perish with the Earth, no matter what.

How do the stars produce energy?

Stars are powered by nuclear fusion in their cores. Over most of their lives, most stars convert hydrogen to helium by a series of nuclear fusion processes; we say stars are "on the main sequence" when they are in this phase.

This was once one of the great mysteries in astronomy, but figuring it out how big things like stars power themselves required us to understand the physics of some of the smallest things -- atoms! That didn't happen until the development of atomic and nuclear theory in the early 20th Century. Physicists could easily work out how much energy the Sun was generating based on its brightness, and they could figure out things like how hot it had to be at the center, leading to the conclusion that some sort of nuclear fusion process must be at work, but exactly what it was and how it worked was a great mystery. The first inkling came from two physicists, Gamow and Weizsäcker, who proposed something called the "proton-proton chain." It was only part of the story; the full details were worked out in 1937 by Hans Bethe and Charles Critchfiled, who figured out that several proton-proton chains together would turn 4 hydrogens atoms into helium. Bethe would receive the 1967 Nobel Prize in Physics for this discovery.

Their ideas explained the observable properties of the Sun, and also made predictions of other properties that should be measurable in the laboratory (in particular the creation of "solar neutrinos" that should be observable on Earth). This interplay of theory (working things out with mathematics) and observational confirmations is one of the hallmarks of how we like to think modern science works, and this is one of the great examples of that model of operation.

What is dark energy?

This is among the most important mysteries in modern astrophysics.

In the late 1990s, in experiments to measure the expansion of the Universe, physicists noticed that many supernovae they were observing were dimmer than expected. After many attempts to understand why this was happening, they reached the inescapable conclusion that that expansion of the Universe was accelerating -- distant galaxies are not only getting farther away from us, but the separation is growing faster and faster with time.

The expansion of the Universe, and the ultimate behaviour of the expansion of the Universe, depends on what is IN the Universe -- sources of gravity. That includes ordinary matter (stuff like what stars and you and I are made of), but it also includes "dark matter" (another unknown substance which emits no light, but produces attractive gravitational force, just like ordinary matter). With the discovery of the accelerated expansion of the Universe, astronomers needed a way to explain what they were seeing, so they added to the mix of stuff in the Universe something that we now called "dark energy." But that's all we've done -- we've added a new bit to the mathematical way we describe Cosmology -- it correctly describes what we see, but that's it. We don't know WHAT it is yet; that's what we are trying to figure out! Sometime in the next decade, NASA is expected to fly a mission called WFIRST that is going to try and see a lot more supernovae to try and help us better understand this mystery.

How does time change near a black hole?

The current modern understanding of gravity used by astronomers and physicists is called "general relativity." In its most intuitive formulation, we think of it not as describing motion under the influence of a gravitational force, but rather as motion resulting from the structure of space and time.

At the heart of general relativity is the idea that massive systems change the structure of space and time, and the result is that motion of other masses change in response to that. One of the curious predictions is that if gravity changes how you move through space (you want to go in a straight line, but gravity bends your path) it must also change how you move through time! This may seem odd, but in careful laboratory tests we find that it is true. You actually depend on this fact every day -- the GPS navigation on your phone only works if you account for the bending of time by Earth's gravity.

On Earth, the time bending effect is small (detectable with technology, but not by me and you personally) because the gravity is weak. Near to a black hole the gravity can be MUCH stronger! The overall effect is the closer you are to a source of gravity, the slower time flows compared to someone far away. So on Earth, clocks tick slower than clocks on the space station. Near a black hole, clocks can tick extremely slow compared to clocks far away. What does this mean in practical terms? Imagine you and your friend visit a black hole in rocketships. Your friend stays comfortably far away, and double dares you to fly your rocket ship down close to the black hole. You fly close, and perhaps only a hour elapses on your watch. When you get back to your friend, a much longer time has passed for them -- your watch ticked slower. The exact amount depends on how strong the gravity got on your journey.

What is the difference between fusion and fission?

These are both nuclear processes that turn the nuclei of one kind of atom into the nuclei of other kinds of atoms. In both cases, energy is released.

FISSION is the breaking up of large atoms into smaller ones. This is famously the process used in the first atomic bombs -- the Trinity bomb, as well as the Hiroshima and Nagasaki bombs were all fission bombs -- they broke up uranium atoms in a sustained chain reaction, releasing enormous destructive energy. Fission is also at the heart of modern nuclear reactors. How quickly the uranium chain reaction is allowed to proceed prevents a destructive release of energy. The first successful such reactor was built right here in Chicago in 1942 (called "Chicago-Pile 1" -- you can visit the original site of the reactor on U Chicago campus, as well as visit the site where the reactor remains are buried, in the Red Gate Woods Forest Preserve).

FUSION is the merging of smaller atomic nuclei to make larger atomic nuclei. The classic example we already mentioned is the fusing of 4 hydrogens to make a helium -- the process that powers the stars. In terms of energy generation on Earth, fusion has always been a desirable goal because the fuel (hydrogen) and by products (helium) are not toxically radioactive. It is, however, a much harder process to initiate and sustain, and has yet to be effectively implemented in the laboratory.

LISA for a Layperson
by Katie Breivik
Spring 2016

Graduate student Katie Breivik studies the formation of pairs of black holes with computer simulations. Her work exhibits the capabilities of a space-based detector called LISA, the "Laser Interferometer Space Antenna" that will be jointly flown by NASA and the European Space Agency near the end of the next decade.

We asked Katie to explain LISA and LISA Pathfinder. (At left, Katie is pictured with CIERA LISA group members Prof. Shane Larson and graduate student Michael Katz, with a model of LISA behind them.)

Please click to view the answers.

What is LISA Pathfinder?

LISA Pathfinder was designed to be a proof of concept for a gravitational wave mission in space. It is a single spacecraft that has two test masses (~2x2x2 inch cube of Platinum-gold) connected by a high powered laser inside of it. (It's kind of like a tiny version of one of the LIGO arms.)

What does LISA Pathfinder do?

There are two key things that LISA Pathfinder was designed to do (and it has pretty much done them!):

(1) Launch two test masses into space, then get them into a perfect free-fall. This enables the same type of measurement as LIGO did to be done in space.
(2) Design a spacecraft that can encase the test masses and shield them from things that could mess up the detection (think dust particles, solar radiation, general space junk). This requires the spacecraft to have micro-thrusters that can give as little thrust as 10^(-6) Newtons (which is really small).

Job one is completed (Woohoo!) and so far, the spacecraft has been doing its shielding job too, so everyone is thinking of LISA Pathfinder as a resounding success. The mission isn't complete yet, but the big hurdles have pretty much all been passed.

Why is LISA Pathfinder important?

One of the biggest risks in building an experiment in space is not knowing how to build it. On the ground, when you build an experiment, if you build it and it ends up not quite working the way you expect, you tear it apart and rebuild little bits of it. Part of LISA Pathfinder's mission is right here on the ground, before we even went into space: learning how to build a LISA style spacecraft.

What is LISA?

LISA is pretty much a giant version of LIGO in space. Where LIGO has arms that are 4km long, LISA's arms are millions of kms long. There are two things that people like to hear about LISA: (1) what will it see? (2) when is it happening?

What does LISA look for?

LISA has a few main sources:
-- Compact binaries (much like the black hole binary that merged that LIGO observed) in the galaxy (and probably nearby galaxies).
-- Super-massive black hole mergers: These are a lot like the LIGO black hole binary, except that instead of ~30 solar mass black holes, super-massive black holes have millions of solar masses and are usually found at the centers of galaxies. In the same way that LIGO saw the black hole merger from very far away, LISA will see these mergers from very, very, very far away. This is because the energy released goes way up when the mass of the black holes merging goes way up.
-- Extreme mass ratio inspirals: These are when a normal mass black hole orbits and subsequently falls into a super-massive black hole. We will see this happening for any black holes that fall into the super-massive black hole at the center of the galaxy.

When is LISA happening?

Right now, the status is that ESA (the European Space agency) has a LISA-like mission: "eLISA" (evolved LISA) scheduled for its next 'big mission' launch in 2034. NASA has expressed interest in working with ESA on eLISA and is in the process of deciding what they will contribute (money, technology, astrophysics research, etc.) This question will (almost certainly) be answered by the end of this decade, when NASA decides what its plan is for the 2020s.

While the timescales seem to be pretty long (2034 is a long time away), I think the gravitational wave community has little doubt that a space-based gravitational wave mission will eventually fly. The science that comes from a space mission really can't be done any other way, so we have to fly it at some point.

In summary, LISA Pathfinder has more or less proved that a gravitational wave detector like LISA is possible in space. We should fly LISA because the science it will do isn't possible with anything other than a space based gravitational wave mission, so it will happen eventually.