What Are Stars?

Most recent answer: 10/22/2007

what is stars?
- Jean Carlos Pino (age 13)
William C. McGinnis school, Perth Amboy , NJ.
That’s an excellent question, Jean.

Stars are gigantic balls of gas, mostly hydrogen gas. There is so much gas and other material that the gravity of this huge gas-ball holds everything together. There is so much gravity that the gas becomes very dense and hot.

Our own sun is a star. Fortunately, we are far enough away from it that the gravity of the gas can’t pull us in. It would be rather unpleasant inside of a star.

The gravity is strong enough that it squeezes the gas together so tight that nuclear fusion occurs. That means that the nuclei (centers) of the atoms get stuck together and ’fuse’. This releases a lot of energy and causes the stars to heat up. The heat works its way from the inside of the star to the surface, and then radiates into space. So what we see of stars is the energy released from the nuclear reactions inside their cores and then radiated from the surface. Since most stars are VERY far away, the light takes anywhere from a few years to millions of years to reach us. So the star light that we see is very old indeed.

There are many different kinds of stars. They come in many sizes and colors. Some stars even orbit around each other. We call these binary stars. If you have any more questions about stars, please ask us to tell you more.

Math Dan

(published on 10/22/2007)

Follow-Up #1: venus

ok but is venus a star? can we see any star in the sky at night? which stars are visible?
- neelkirloskar (age 14)
Venus is very definitely not a star in the scientific sense of the word. It's just an ordinary planet. The reason you can see it so well is that it's closer to the Sun than us, about our size, and fairly reflective.  So you see lots of sunlight reflected off it.

I'm not quite sure how to answer the question about which stars are visible. Whether a star is visible depends both on how much light it gives off and how far away it is.

Mike W.

The number of stars ’visible to the naked eye’ varies.  In isolated rural areas or at sea it’s several thousands.  In well lighted urban areas it might be 100 or less. 

(published on 10/22/2007)

Follow-Up #2: other solar systems

I have a quick question about stars and galaxies. Since you say that the stars are a sun just like ours, does that mean that there is millions of galaxies with their own sun and set of planets just like ours?
- Ogi (age 23)
We don't have the instruments yet to directly detect smallish planets like our own around other stars. However, from what we know about the universe, the answer is almost certainly yes, at least as far as the general physical features go. In fact it's quite likely that there are more than a million million million earth-like planets, just counting the visible part of the universe. As for what fraction of the planets have life on them, we really have no idea, although some people have very strong opinions on the subject.

Mike W.

(published on 10/22/2007)

Follow-Up #3: sun and stars

im having a hard time believing that the stars are really suns. So from a stars distance, does our sun look like a tiny little star?
- Christine (age 16)

Mike W.

(published on 04/03/2008)

Follow-Up #4: stars and planets

what i can answer to my 4 years old son to his question,so he can understand: "Mom, what r the stars?" and "What is the difference between star and planet?"?
- nikol (age 31)
I guess you could tell him that the Sun is a star, and that many other stars are similar to the Sun but they are very far away and so only appear as faint dots. The earth is a planet, and some of the other planets are similar to the Earth. Planets are just clumps of stuff (like the Earth) and give off no visible light, except whatever happens to bounce off them from a nearby star. Stars are extremely hot and therefore glow, like fire or an incandescent light bulb. What keeps them hot are nuclear reactions, kind of like in a hydrogen bomb, only running steadily rather than in a short burst.

There are other types of stars, and somewhat different planets (e.g. bigger colder ones like Jupiter) but I think this description may help your son get started.

Mike W.

(published on 05/09/2008)

Follow-Up #5: other planets

I know that stars are really suns burning away in the distance but...if there are so many suns then shouldn't there be planets they go with? It just seems strange that why do we have our own sun when there are loads more suns in the sky then does that mean there are more planets that just havent been found yet ? Thanks Rhiannon xxxxx
- Rhiannon Jones (age 16)
wales (wrexham)
Nice questions. You're right- there are lots of planets around other stars. Since planets are so much smaller and dimmer than stars, they're hard to see. However, in recent years a number of big planets have been found around nearby stars. As techniques have improved, smaller planets have also been found. So there are lots of planets around.

We really have no idea what fraction of these other planets have life, because we don't know how likely life is to get started even on a hospitable planet.

Mike W,

(published on 07/10/2008)

Follow-Up #6: stars and black holes

what do stars have to do with black holes
- tom barnett (age 12)
brightlingsea colchester essex england
There are several connections between visible stars and black holes. Large stars collapse to form black holes when they use up their nuclear fuel.  Often one star will be in orbit around a black hole. Stars cluster in groups called galaxies, which often have very large black holes near the center. Stars sometimes get swallowed  up by black holes.

There are probably other connections, but those come to mind.

Mike W.

(published on 07/14/2009)

Follow-Up #7: What are shooting stars?

what are the shooting stars and how does that happen? thanks
- ravi (age 13)
london, uk
Well, they are not really stars at all but tiny bits of micro-meteorites that enter the earth's atmosphere and are heated to incandescence by the friction of the air. They get white hot and make a streak of light through the sky.   They usually get burned up before they reach the ground but a particularly large one can make it all the way.  Several times a year the earth passes through large clouds of meteor dust and an exceptional display of shooting stars can be seen.  One good example is the Perseids shower which occurs around August 12th this year.    Take a look at for more details.


(published on 07/15/2009)

Follow-Up #8: Are the stars we see at night all in our Solar system?

Are the stars we see at night all in our Solar system, or galaxy or perhaps even further / closer. ?
- Tom wells (age 16)
United kingdom
All of the stars you can see with the naked eye are from our own galaxy, the Milky Way. On a moonless night you can see several galaxies, such as the Andromeda, as faint smudges but you can't discern individual stars without a big telescope. 


(published on 07/25/2009)

Follow-Up #9: Do stars flicker different colors?

Do the stars flicker in different colors or is that just our eyes playing tricks on us?
- patrick mcmahan (age 26)
s.s. tx. usa

Hi Patrick,

I'm not sure if you're asking if a given star may flicker (or "twinkle") different colors, or if actual, physical color varies from one star to the next.

If it's the former, not exactly. The combination of dust in the Earth's atmosphere and less-than-uniform atmospheric density cause light from distant stars to refract on its way to our eyes, which may result in apparent "twinkling" of various colors. If you'd like to know more about this, please let us know!

However, I'm going to assume the latter -- that you're asking if the stars themselves are actually different colors. In this case, the answer is yes. There are several reasons for this, but temperature at the surface of the star is by far the most important factor. Generally speaking, stars obey the following temperature rules, with hotter stars appearing bluer.

Of course, composition matters too! If you've ever taken an introductory chemistry course, you've likely had an opportunity to watch different elements burn their respective colors. Strontium burns a deep, deep red, barium a brilliant green, potassium a stunning purple, and soon... This is the same phenomenon which allows us to have elaborate fireworks displays every Fourth of July! Stars are by and large composed of the same basic elements (namely, hydrogen and helium) but trace elements like oxygen, carbon, nitrogen, silicon, magnesium, neon, iron and sulfur -- whose concentrations depend very much on the environment in which the star was born -- may add their own colors of combustion to the mix.

It's also worth mentioning that the Doppler Effect means that we see many stars as "red-shifted" (of longer, redder wavelengths than they actually are) because they're moving away from us as the Universe expands. This motion causes the wavelengths of a star's light to spread out behind it, as seen below.

(image source: )


(published on 08/20/2009)

Follow-Up #10: gravity and stars

In the first question that was presented, you stated that stars are gases mainly compressed by gravity. Where does this gravity come from and it is just a bunch of covalent/ionic or metallic bonds? Not sure if i will read your reply so could u please send it to my email
- Edwin A (age 18)
We only do Web postings, not email answers.
Anyway, gravity doesn't consist of those other types of bonds, which all come from electromagnetic forces. Gravity follows very different laws than electromagnetism does. For example, gravity is always attractive.

There are attempts to at least understand gravity in the same framework as the other forces, but so far those haven't been fully successful. At the level of ordinary experience, gravity is a completely separate force.

Mike W.

(published on 09/18/2009)

Follow-Up #11: At what stage is the evolution of our Sun?

I have basic knowledge about astronomy. I always wondered though. What age stage our sun is on as a star? like supernova whitedwarf etc...
- Tessa (age 20)
Our sun, a very ordinary star, is living comfortably in middle age as it travels along the road of the Main Sequence evolution line.  It was born about 4.5 billion years ago and will live another 4 or 5 billion years before old age sets in.  At this time much of the fuel in the sun will have been burned and the sun will turn first into a red giant and then shrink into a white dwarf and senility.  During the red giant phase, the sun's radius increases and will actually engulf the inner planets including the earth. After that it will collapse into a very small whitish star. The Sun is not massive enough to become a supernova or a black hole.
See    for some interesting details.


(published on 10/09/2009)

Follow-Up #12: Will life end in 4 billion years?

From your last answer, does that mean life will end in about 4-5 billion years from now when sun will stop shining.
- Arun
Chicago, il
I'm afraid so, at least on the planet earth.  But a billion years is a really, really long time.


(published on 12/29/2009)

Follow-Up #13: Is that the Milky Way?

I went for a holiday and during the trip, I saw this faint band of white glowing. I think it's the milky way. I know that the milky way is actually the galaxy Earth is in. But which part of the milky way was I seeing then? Is there a certain arm of the milky way that is more visible than others? Is it correct to assume the band that I saw is the arm of the milky way the Earth is in? (orion arm?)
- aly (age 20)
If you live in or near a large city like Singapore the light pollution from the city lights obscures much of the beauty of the heavens.  When I was a boy I lived on a farm far away from city lights.  I remember well the summer nights when we would take a blanket outside, lie in the grass and marvel at the wonders of the universe.  We would see the Milky Way, count shooting stars (actually micro meteorites entering the atmosphere), try to figure out the constellations and the planets, and dream of flying to Mars
What you describe seeing was probably the Milky Way. It is the edge-on view of our own galaxy.  It does pass through the constellation Orion. The center of the galaxy lies toward the constellation Sagittarius.  Our solar system lies about 3/4 of the way to the outer edge.   

There is a wealth of information on the web: just google 'Milky Way' and you will be inundated with web sites. 


(published on 07/01/2010)

Follow-Up #14: Bright stars in the North?

i just went out earlier and i saw about 5 -6 stars over my house toward the north there was a very very very bright star brighter than all the rest is that the northern star or just a star?
- Charlii (age 17)
United Kingdom
According to the sky map for September 26, 2010, (http://www.skymaps.com/skymaps/tesmn1009.pdf) Deneb, one of the largest known super giants and the brightest star in the Cygnus constellation was visible to the naked eye in the northern sky. It was either this or Capella, the sixth brightest star further off to the North-Eastern sky. But just to clarify, it is a common misconception that the North Star "Polaris" is the brightest star in the sky. Polaris is special though, the apex (imaginary line drawn straight up from any given position on Earth)from the North Pole goes up to Polaris, so at your home in the United Kingdom, Polaris will always be in the Northern sky. -Chris Freund

(published on 09/26/2010)

Follow-Up #15: Is the sun ordinary?

You guys say that our sun is an ordinary star, however I've read that the great majority of stars inside the Milky Way are red dwarfs. In that case, wouldn't red dwarfs be ordinary stars rather than sun-like stars since they form the majority of stars? Isn't be a bit self-centered for humans to rate their own star an ordinary one as if anything that doesn't fit their immediate environnement cannot be ordinary? Quote from Wikipedia on red dwarfs : "They constitute the vast majority of stars and have a mass of less than one-half of that of the Sun (down to about 0.075 solar masses, which are brown dwarfs) and a surface temperature of less than 4,000 K." I'd like to have your opinion on that!
- Anonymous
OK, the Sun isn't the most common type of star. It's ordinary in the sense that it's on the "main sequence" of stellar evolution, and not a particularly rare extreme.  We didn't mean to be Solist and certainly didn't mean to imply that there was anything weird about red dwarfs. We apologize to any readers who may dwell on those more common stars.

Mike W.

(published on 02/11/2011)

Follow-Up #16: Observing Dead Stars

when we look at the sky..what proportion of the stars do not exist as stars anymore ie the light hits the retina, but the star itself c does not emit light anymore
- steviep (age 57y)
staffs uk
Hi Stevie,

We often hear about how, since the speed of light is a constant, when we look into space we're looking into the past. However, the answer to your question, how many stars that we see are no longer acting as stars, may surprise you! For simplicity, let's assume that we're defining a star as something that is fusing elements together to make heat and light, so that if a star becomes a white dwarf (which still gives off some light) we will say it is dead.

The answer then depends on how you're looking at the sky! If you're looking with the naked eye, then all the stars you are seeing are our nearest neighbors. The Milky Way galaxy is about 100,000 light years across, and we are seeing the closest stars: most visible to the naked-eye stars are within 4,000 light years. The average lifetime of a star depends on how big it is: the brightest, most massive stars last for about 10 million years; stars like our sun can burn for about 10 billion years, and the dimmest, reddest stars can shine for about 100 billion years! (This last number is theoretical---since the universe is only 13.7 billion years old we haven't seen any of these die out yet.) Since we are only looking back 4,000 years, very few of these stars will be in this last stage of their lives, crossing this threshold during the time light travels from them to us. As a result, probably only a handful of stars visible at night are no longer emitting light.

The answer changes if you have a telescope with you though. With a moderate-sized telescope, you can see the Andromeda Galaxy, just over 2 million light years away. Based on the lifetimes mentioned earlier, in looking at the Andromeda galaxy, 20 percent of the brightest stars are no longer shining. This number is much smaller for the dimmer stars, since they last longer; the dimmest stars are all still there! The dominant type of star in our universe is the dim star, so even in this case probably only hundredths of one percent of stars in the Andromeda Galaxy are not shining any more.

However, if you had a giant telescope, like the in Hawaii or the proposed to be built in Chile, you could observe some of the galaxies farthest away from us: some of these galaxies emitted the light we're seeing 13 billion years ago! At this distance, all the brightest and sun-like stars have stopped emitting light. There are still many dim, red stars from this time still alive, but since these stars are so dim, we don't see them. The light we actually see is primarily from the brightest stars, so here, the vast majority of stars we see are actually now dead.

Thanks for the great question!
Ben M.

(published on 02/18/2011)

Follow-Up #17: If all stars are suns, then why don't we call them suns?

If all stars are suns, then why don't we call them suns.
- Bernard (age 62)
From Romeo and Juliet:  "What's in a name? That which we call a rose by any other name would smell as sweet" 

Stars can be suns, if they have inhabitable planets that have cognitive life and they decide to call its life giving radiation their sun.  Our very own sun is a star very similar to millions of the stars that we see in our telescopes.  What's the difference?


(published on 03/07/2011)

Follow-Up #18: Does a star have an atmosphere?

does a star have an atmosphere?
- abraham (age 24)
Sort of, but it is not an atmosphere like we have here on Earth.  The Sun's atmosphere, sometimes called a chromosphere or photosphere, doesn't have much oxygen in it.  It is mostly extremely hot gasses like hydrogen.  You couldn't breath that kind of stuff.


(published on 04/18/2011)

Follow-Up #19: seeing star history

I had the same question and am glad you answered, however, I am still a little confused. You said that when viewing stars through a giant telescope you are seeing the stars as they looked 13 million years ago? Is this correctly understood? So, following this thinking if I am correct, if we wanted to see how those stars look today we would have to wait millions of years? So we can never really see what the present looks like? Also, I understand the basics of the speed of light and how this affects time and how we see things but is there a simple way to explain how it works, like for a child? And, why does the size of the telescope change how far back in time we are seeing a star? And my last question, does this also apply to planets? If I were looking through a giant telescope at say jupiter, would I be seeing present day jupiter or jupiter as it was at some point in the past? Thanks, this is confusing stuff but glad you can help clear it all up.
- Christina (age 30)
Santa Rosa, CA USA
that's a lot of nice questions.

1. Actually, the oldest ones are about 13 billion years old (as Dr. Evil would realize.)

2. So yes, we'd have to wait a very long time to see how these stars are "now". I use "now" in quotes because there is no unique way of saying what constitutes the same time as now when you are discussing things very far away. Probably the best convention would be "at 13.7 billion years after the Big Bang in their own local time".

3. I actually don't know a good way to explain even Special Relativity to a very young child. Maybe somebody else does.

4. The size of the telescope doesn't have any effect on when in its history you see a particular object. It's just that bigger telescopes let you see more objects, farther away, so you can see ones that were very old. You're seeing Jupiter just half hour to about an hour old (depending on how close it is to earth) either by eye or by telescope.

Mike W.

(published on 11/15/2011)

Follow-Up #20: twinkling stars

I know that Stars are suns and alot of the above questions helped me win a debate with my girlfriend, so thanks for the ammunition. Another debate we are having are about twinkling stars, she seems to think they're aeroplanes or helicopters... i know they're stars but why do they twinkle? and why is it only a select few? thanks
- Conor (age 22)
Belfast, N.Ireland
Stars twinkle because the light from them passes through the atmosphere, which has density variations which change in time. That's like passing through a constantly shifting set of lenses. It makes the image move  around just a little. I think it happens with all stars, unless you're viewing from space. Maybe it's most noticeable with the brighter ones.

Mike W.

(published on 05/16/2013)

Follow-Up #21: livable planets?

hi, you said previously that around every star(sun)there are planets like earth do you know if any of these planets can sustain life like our earth? and is it true a new planet called kepler has been found that is like earth?
- Daniel (age 17)
Norwich Norfolk England

We didn't say that every star has earth-like planets. Many stars have planets. Some fair fraction of those are likely to be mid-temperature planets maybe somewhat like the Earth. There are so many stars, maybe an infinite number in the universe as a whole, that there are bound to be many Earth-like planets even if most stars don't have them. There are bound to be some that could sustain life. Unless the total number of planets is infinite, it's hard to say with confidence if any others actually do sustain life.

As for the planet "Kepler", we only know what we read on Wikipedia: .

Mike W.

(published on 09/02/2013)

Follow-Up #22: life on other planets

First, thank you for educating me. I would like to know if the current rate of astronomical discovery continues, when do you think it would be possible to discover new life sustaining solar systems? Or perhaps, when could we have visual evidence of life in other solar systems? I know that your answer is theoretical, but you are far more knowledgeable in this regard than I.
- Rebbecca Bittigar (age 33)
South Carolina

Hi Rebbecca- I've got to disagree with one point you make, namely that we are more knowledgeable than you about how frequently life appears throughout the universe. If it turned out that there was life near one star out of a thousand, I'd be only mildly surprised. If the turned out that there was life near less than one star per 1016, I'd not really be surprised either. If it's the former, then expect to find some evidence in your lifetime. If it's the latter, find other interests. So we really don't have a clue.

Why the great uncertainty? For life to get going, you need some sort of replicator, an entity (chemical, for life as we know it) that can trigger formation of another entity that shares a lot in common with it. Ordinary crystals have that particular property, so the replicator needs something else too, that a range of small changes are possible giving a big variety of possible results. That allows evolution to get going. The problem is that the simplest replicators we know of are already complicated enough to be very unlikely to form directly just by chemical accident even given a nice big planet and millions of years. So the start of life may be a very rare event. On the other hand, there may have been much simpler replicators from which the simplest ones we know evolved. Maybe the simpler ones can form pretty easily, so that life typically gets going on a friendly planet. 

Some people argue that since life got going on Earth very quickly after the Earth cooled enough to let water condense, that's evidence that life gets started easily. I have my doubts about that. Getting life started requires keeping a collection of organic molecules including some fairly big polymers together interacting for a while. That should have been much easier when there were just small wet spots on a fairly dry Earth, say in little cracks and crevices in rocks, rather than large oceans for the molecules to wash into. That means that there may have been one best window for starting life, just before the oceans formed. That the start seems to have occurred here in just that window then unfortunately would give us no information of whether such starts are common or rare.

Mike W.

(published on 01/13/2016)