Questions and Answers about Stars

1. If a close enough star exploded would we ever be able to hear or even feel the explosion?

The answer is maybe. †Maybe, because if an exploding star like a "supernova" were close enough to affect us directly, it would probably kill all life on earth before anyone could feel anything! †But you can relax because there aren't any stars in our galactic neighborhood that are likely to explode.

But say there a Blue Giant star (which typically ends its life as a supernova) was located as close as Alpha Centauri. That's 4 light years away (20 trillion miles), There aren't any Blue Giants anywhere near that close to us, so this is completely hypothetical.

If that Blue Giant became a supernova, it would send out neutrinos, high energy electrons and protons, gamma rays, x-rays, and intense visible light in all directions. There isn't any sound in space, (contrary to what you might have heard in Star Wars), so any sound effects on earth would occur after the particles and light rays reached the earth's atmosphere.

Only a very small fraction of these particles and rays would strike the earth, but at the "close" distance of 4 light years, it might be enough to heat our atmosphere to very high temperatures, cooking everything on the side of earth facing the star in a few minutes. †People in the high northern hemisphere (northern US, Canada, Europe, Russia, etc) wouldn't be affected immediately, because Centaurus is below the horizon. †But the heated air on the Centaurus-facing side of the earth would probably cause a great shock wave that would race around the earth. †We'd all hear an enormous explosion from the shockwave before getting smashed to smithereens. †After that, the earth and other planets would be turned to cinders. It's a violent universe out there but, fortunately for us, all the violence is far enough away in space and time that we don't have to worry much about it. †Our galaxy, after all, is a big place.

Ed Schmahl

2. If most starts in the universe are binary then why is ours alone. Do the other plants (not in our solar system) are they binary?

Actually, only about 1/2 of the stars in our galaxy are binaries (we're not really sure about stars in other galaxies, but the binary fraction is probably similar in galaxies like the Milky Way). †That means one way you could look at the answer to your question is that we had a 50-50 chance of being in a binary system, and it turns out we ended up with a single star. †But astronomers don't like leaving things to chance. There may be a good reason why the Sun is a single star. †If the Sun had a companion that was as close as any of the planets (say, within the orbit of Pluto at 40 AU), then its gravity would produce strong perturbations in the orbits of the planets, and our solar system might not be able to survive as long as it has (4 billion years). †Thus, it may be less likely to have planetary systems around binary stars.

The extra-solar planets we know are also mostly around single stars (or wide binaries). †But these stars were selected because they were single stars, we haven't looked for planets around close binaries yet so we don't really know if there are any.

Jim Stone

3. Have we in the history of our world ever witnessed a star in a familiar constellation burn out or blow up? †Meaning one night when looking up at our favorite or familiar constellation one of the stars that make up that constellation is missing one of its stars. †Is it possible or ever going to happen?

There have been several supernova explosions in history which were bright enough to see just by looking in the sky. †In most cases the stars were too faint to see without a telescope before they blew up. Here are three from the last 1000 years:

†1054 A.D. Astronomers in China saw a bright light in the constellation we call Taurus the Bull. †You can look at the expanding cloud of debris today using a small telescope. †The cloud is between the horns of the bull. †A picture and a diagram showing where to look are at


†1604 A.D. A supernova in Ophiuchus was seen by Johannes Kepler. †1987 A.D. One of the massive stars in our neighbor galaxy the Large †††Magellanic Cloud exploded. †Even though massive stars take a few Million years to run out of fuel and blow up, there are so many of them that a nearby one dies every few hundred years.

A lot of the stars in the familiar constellations are smaller stars like our Sun. †These use their fuel more slowly and live for Billions of years. †They die by swelling up, sending out puffs of gas, and then fading away. †This takes maybe a hundred thousand years, so no-one has watched a star go through the whole death process. †However the puffs of gas can be seen and I think they look rather pretty. †A picture of one is at

Neal Turner

4. I was wondering, how do you estimate the total number of stars in a galaxy?

We can measure the brightness of the galaxy, and then use our knowledge of the brightness of stars to figure out how many are required to produce the measured brightness. †But, there are a number of issues to consider. †If all stars are like the sun, then we just divide the total brightness by the sun's brightness. †Of course all stars are not like the sun. †So we have to account for the different kinds of stars (that have different brightnesses). †We also have to account for extinction. †Not all the light that a galaxy emits reaches us due to dust obscuration.

So, as you can tell, it is a tricky business to try and count the number of stars. †But for the Milky Way galaxy, we can see (resolve) most of the stars. †We also have a pretty good idea on the amount of dust in the galaxy, so we can account for extinction. †So counting the stars is not as hard. †We can look at a small region of the galaxy to get the number of starts per volume (density of stars). †We can then multiply that value by the volume of the galaxy. †Of course, we have to make sure that the patch we are looking at is representative of the whole galaxy. Astronomers estimate the number of stars in the galaxy to be about 10^11.

Rahul Shetty

5. I've always wondered why some pulsars spin faster than others? I donít understand the theory of that.

You can break your question up into two questions:

1. How does a pulsar start spinning, or spin up?

2. What determines how rapidly a pulsar's spin slows down?

Let's start with the second question. †A pulsar is a neutron star, and a neutron star has a strong magnetic field. †If the star spins, so does the magnetic field, and you can show that a spinning magnetic field takes away energy and spin. †Therefore, a spinning magnetic object will slow down, and the stronger the field is the faster it will slow down. †Therefore, a partial answer to your question is that some neutron stars have stronger magnetic fields than others (for reasons we don't completely understand), so that even if all pulsars were to start out with the same spin rate, some would slow down faster than others and therefore spin differently after a time.

Now the first question. †There appear to be two ways for a pulsar to get its spin. †The first is that when a neutron star is produced by the collapse of a massive star (much larger than our Sun), it has some initial spin. †As you might expect, different stars collapse differently, so it's not surprising that this initial spin would vary between pulsars. †The second way is that if a neutron star is in a binary with another star and some gas from the other star falls onto the neutron star, this can spin up the neutron star (think of pushing a merry-go-round to get it going). †This process is known as "recycling". This turns out to produce the fastest pulsars, called millisecond pulsars because they spin once every few milliseconds (the record holder spins around once every 1.5 ms, or 0.0015 seconds!). †The millisecond pulsars also turn out to have weak magnetic fields, so they don't slow down much.

Cole Miller

6. The explanation our book gives for the possibility of gamma-ray bursts is that in a binary system both stars become neutron stars and collide into each other. My question is with the theory of stellar evolution already in place, how is it possible for a binary system to have two neutron stars at the same time?

The basic gist of stellar evolution theory (for a single star) is based on how massive a star is. Stars of a certain mass will form neutron stars. If a binary star forms, the stars could be "identical" twins or they could be very different. How each then evolves depends not only on its original mass, but how close the other star is. Binary stars that are widely separated (>1000 stellar radii, Astronomy Today, 2nd ed, Chaisson/McMillan), will evolve more or less independently (i.e., as if they were single stars). But stars that are closer will in fact influence the development of the other.

Surrounding each star is a tear shaped region called the Roche lobe. Usually, the stars are completely within their lobe, if one is more massive so that they evolve at different rates, then one will eventually become a giant and fill its Roche lobe. Then some of the material can start to flow over into the other Roche lobe and onto the other star. This has the effect of changing the second star's mass. You can have a whole series of back and forth transfers. It all depends on the starting masses and distance between the two. In any case it is conceivable to have a binary star whose components eventually become neutron stars. Another theory is that a single neutron star flying through space ('ejected' from its previous location by the supernova that formed it) can run into a binary made of two low mass stars, ejecting one of them and taking its place. This idea allows for a binary with one neutron star without having to explain how the other star "survived" the supernova explosion that created the neutron star.

Anyway, while it is not easy to explain how a binary system has two neutron stars in it, it is possible. It's just one of the more exotic endings that might not be well described in the textbooks, at least not yet! †With the idea that GRBs might be caused by two orbiting neutron stars running into each other, understanding how the pair formed (either evolved or by capture) becomes very important. And while we may not be able to describe the evolution very well, we have seen NS-NS binaries so they are more than theoretical objects.

Elizabeth Warner

7. What is the evidence that protostars form after a collision between gas clouds?

Most of places in the Universe have very low density, which means very little material is distributed in large space. †Stars, on the contrary, have very high density and temperature to ignite the nuclear burning to produce light. †Therefore, the main principle to form stars in space is to bring enough matter together. There are two ways to bring matter together: simultaneous star formation and stimulated star formation. Simultaneous star formation refers to a single cloud that collapses into stars due to its own gravity. †Stimulate star formation rely on external force to push mass together. Cloud-cloud collision is one kind of stimulate star formation process.

This is a good question, because a good theory does not only rely on the imagination. It needs evidence. †One way to prove that cloud collisions do produce protostars is to catch the process in act. †For example, W49 North, a molecular cloud in our galaxy, consists of several simple velocity components and a group of massive protostars in the center, which matches the picture of the star formation induced by cloud collisions. †On the other hand, given the mass distribution, one can calculate how many stars can form in a galaxy with simultaneous star formation. However, the number of the observed massive star clusters (OB association) exceeds the expectation value for pure simultaneous star formation. †Computer simulations also show that star formation through cloud collisions is common in the condition of a typical galaxy. †Therefore, we believe that certain amount of stars (most of them are massive stars) do form through gas cloud collisions.

Cole Miller

8. My question is how it is possible to determine the number of stars in a galaxy when there is such a wide array of stars with differing masses? Mr. Edwards showed us a sample model of how to find the total mass of a galaxy, but I did not understand how that translated into the number of stars present.

To obtain the total number of stars, we need to know the numbers of different kinds of stars. †We can get an idea of what kind of stars there are in a patch of the sky by looking at the light from the stars. †The light will have absorption and emission lines that are specific to certain types of stars. †Once we know the number of stars in a small patch of the sky, we can multiply that number by the volume of the galaxy to get the total number. †Of course, we have to assume that the small patch we are looking at is representative of the whole galaxy. † The total mass of the galaxy generally does not translate into the number of stars, because of the existence of dark matter.

Rahul Shetty

9. If its not to much trouble I am curious about the speed of stars. My second question is what is the fastest star? Of course a shooting star is probably faster than a non shooting star, but which star that's not shooting is the fastest? Thank you for your time and i appreciate you answering my question.

You ask a very good question, inquiring about the speed of stars. Many people look at the stars at night and they appear to not move over the course of a night, so people think that they don't move or that they must move very slowly. However, this is very far from the truth! All stars are in motion. Some stars move around each other, similar to how gravity causes the Earth to revolve around the sun. When two stars orbit around each other, it is called a binary system. However, this is not the only motion. Collections of stars make up various types of galaxies and the stars move around within the collection. For example, our galaxy, the Milky Way, is thought to be a spiral disk. The disk is spinning, which means that the thousands upon thousands of stars in our galaxy are moving generally in a circular motion around the center of our galaxy. Our sun is moving approximtely 250 km/s around the center of the Milky Way. This is a speed of over 500,000 miles per hour!!!!

Think about when your family takes a trip and is driving on the highway. You look to your right, and a little kid in the backseat of the car in the next lane waves at you. At this moment, it appears as though this car is not moving. It is only when you look at the trees behind the car that you realize that both cars are in fact moving very fast. It is like this with the speed of stars in astronomy. Some stars near our position in the Milky Way may appear not to be moving very fast relative to our star, the Sun, but they are still moving in the galaxy.

I do not know what the fastest star is! The fastest stars that I could find are two white dwarf stars which are revolving around each other. They orbit each other once per year. In addition to this speed of motion around each other, they are also zooming around the galaxy. This star system was deteced with the "Very Large Telescope" in Chile and is referred to as RX J08906.3+1527. I've also heard that faster stars may be found in systems of 3 stars where two of them "kicked" the third one out of the system. This is called a "triple system". However, you should hunt and see what you find to be the fastest star!

Shooting stars appear to move faster than non shooting stars when you are looking at the sky at night. However, shooting stars are not really stars at all! Shooting stars are actually small pieces of rock which burn when they enter the Earth's atmosphere. Many people think that space is empty, but it is not! If the space rock is large enough, it can fall all the way to the Earth's ground. Maybe one day you will find a rock that fell out of the sky! There was a funny story that happened several years ago where a lady went to her garage in the morning and found her car's hood had been bashed. She called the cops to make a report so they could find the troublemaker. After some investigation, they found the culprit... a "shooting star"... resting quietly beneath the hood of her car!

Take care,

Ashley Zauderer

10. What is the effect on the Earth of a collision of two nearby stars?

First, I would note that physical collisions between stars occur frequently in dense star clusters via close encounters between two single stars or during strong dynamical encounters involving binary stars. Such collisions are more rare in the vicinity of our star (the sun). The effects of such collisions, such as mass loss, energy loss or coalescence of the colliding stars, depends on such factors and the masses of the stars, the impact parameter (for instance whether the collision is head-on or grazing) and angular momentum and relative velocity. One scenario may be that two stars initially become a binary pair that later violently colaesce during a periastron passage. Coalescence may in some cases lead to the formation of a massive star that eventually becomes a supernova. Nearly head-on collisions may lead to high velocity jets perpendicular to the collision axis with increasing mass loss as the impact velocity increases relative to escape velocity at the surface. While a stellar collision may even lead to gamma ray bursts if the collision results in the formation fo a neutron star, the effect on the earth of a nearby collision is not likely to be noticeable. Even a shock, if it reached the Heliopause, would probably not greatly affect the solar system.

Rosemary Killen

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