Ask Us

You might want to check through our sister site, Imagine the Universe!. And there's a star finder at the University of Turku in Finland.

Dr. Louis Barbier and Beth Barbier

A long time ago, I had heard that if you had one grain of sand for every star in the sky, you would have more sand than there is on Earth's beaches. How big would a bag have to be to hold that much sand?

This is by no means an easy question, and it is a profound one that challenges our imagination. It also reminds me of the famous German children's song "Weisst du wieviel Sternlein stehen?" (Do you know how many stars there are?)

To estimate the number of stars in the Universe (I assume this is what you mean), we have to be clear about a few basic definitions first. If we talk about the "Universe", we should restrict ourselves to the "observable" Universe. Yes, there is a basic restriction as to what we can observe, no matter how much we try. From the latest cosmic background images, we know that the Universe is 13.7 billion years old. Therefore, we will never see any stars or galaxies more distant from us than 13.7 billion light years. This sets the volume of the "observable Universe" as a sphere with radius 13.7 billion light years around us. This, of course, does not mean we are in the center of the Universe, just in the center of the portion that we can observe.

Now we have to make couple of basic assumptions:

1. Galaxies are evenly distributed throughout the Universe (locally clumped in galaxy clusters though). The Hubble deep sky image (which you mentioned), taken in a region with no known foreground stars, has indeed shown to us that there are as many galaxies at the largest distances as there are close to us.
2. Typical galaxies have about the same numbers of stars, on average. Actual numbers in individual galaxies can vary widely. We talk about an average here. Even, if galaxies looked different in the past (another result from the Hubble image), they formed into a similar set of galaxies, with a similar number of stars as the ones we see close to us.

Now, working with those assumptions:

1. Let's assume the large Virgo cluster of galaxies is typical. It contains about 2000 galaxies. Let's say a typical cluster has 1000. Now Virgo is about 100 million light years away from us (another famous Hubble result). Let's assume that galaxy clusters are typically at a distance from the nearest neighbors of 100 million light years and make the assumption that they are distributed at the centers of 100 million light year sized cubes tightly packed (to simplify our calculation). When we fit these cubes into the volume of the observable Universe, this gives us 10 billion (or 10¹⁰) galaxies.
2. Our Milky Way galaxy and our neighbor, the Andromeda galaxy, both have about 100 billion stars. There are some larger galaxies, but also many smaller ones. Let's assume this is a typical size.

Multiplying 1. by 2. makes 1000 billion billion stars. This is the number of sand grains that you need to pile up. A typical sand grain has a size, let's say, of 0.5 mm. Again let's assume these are little cubes (for the sake of simplicity). The collection of as many sand grains as there are stars in the sky would sum up to a sphere with a radius of about 3 km, i.e. the typical size of a comet nucleus, but not as large as a substantial fraction of our moon.

Even if our estimate for the number of stars were off by a factor of 10, the radius of the sand grain aggregate would only change approximately by a factor of 2, because volume requires the cube of the size.

Dr. Eberhard Moebius
(January 2005)

Stars do give off light, that's why we can see them far away. The Sun, which is just an ordinary star, gives off the light that allows life to exist on Earth. Stars give off light the same way the filament in a light bulb does. Anything that is hot will glow. Cool stars glow red, stars like the Sun glow yellow, and really hot stars glow white or even blue-white.

Dr. Eric Christian
(September 2001)

You can see one star during the day -- the Sun! But because the sky is so bright (due to the Sun being bright), other stars are not visible. On the Moon, if you shield the Sun with your hand and let your eyes dark-adjust, you can see stars during the "day".

Dr. Eric Christian
(August 2000)

Just as the Earth's atmosphere makes the sun redder -- which is more noticeable at sunrise and sunset when the sun passes through more atmosphere, interstellar dust and gas make stars appear redder the farther away from us they are. You can research "interstellar reddening", which is the official term.

Dr. Eric Christian
(May 2011)

No, I hear that in space the stars look wonderful, bright (although not twinkling) and very clear. What has probably caused some of this confusion is that in the typical photo or video image from space, there aren't any stars. This is because the stars are much dimmer than the astronaut, Moon, space station, or whatever the image is been taken of. It is extremely hard to get the exposure correct to show the stars. Luckily, the human eye handles the different light levels much better than a camera does.

Dr. Eric Christian
(July 2001)

No, it doesn't. Your ability to see stars is mostly determined by the amount of light pollution nearby, generally from man-made lights.

Dr. Louis Barbier
(November 2002)

The best write-up of this I know of is in the Imagine the Universe! Web site.

Dr. Eric Christian

A star's energy comes from the combining of light elements into heavier elements in a process known as fusion, or "nuclear burning". It is generally believed that most of the elements in the Universe heavier than helium are created, or synthesized, in stars when lighter nuclei fuse to make heavier nuclei. The process is called nucleosynthesis. You can read more about nucleosynthesis on our Web page.

Beth Barbier

Stars like the Sun burn hydrogen into helium in their centers during the main-sequence phase, but eventually there is not enough hydrogen left in the center to provide the necessary radiation pressure to balance gravity. The center of the star thus contracts until it is hot enough for helium to be converted into carbon. The hydrogen in a shell continues to burn into helium, but the outer layers of the star have to expand in order to conserve energy. This makes the star appear brighter and cooler, and it becomes a red giant.

During the red giant phase, a star often loses a lot of its outer layers which are blown away by the radiation coming from below. Eventually, in the more massive stars of the group, the carbon may burn to even heavier elements, but our Sun will never really get past carbon. Eventually the energy generation will fizzle out and the star will collapse to a white dwarf.

(May 2000)

If stars burn hydrogen, won't all of the hydrogen in the Universe eventually be used up? Then no new stars could be created.

Stars are not 100% efficient when burning hydrogen, so it is thought that there will still be some hydrogen around, but it won't be in the right form (dense gaseous clouds) for star formation. But eventually, the Universe will get to where no new stars are formed.

Dr. Eric Christian
(August 2000)

Based on estimates of a star's mass and composition, theories can explain how stars evolve. Observations of many stars of the same class can be used to confirm these theories. Any particular star can be observed and its age determined based on these theories of stellar evolution.

Dr. Louis Barbier

Roughly, the phases of stars are:

gas cloud -> collapse to protostar -> hydrogen fusion (the star is now called a "main sequence" star) -> heavier element fusion (star is now in "giant" phase, usually a red giant) -> burnout (insufficient nuclear fusion to sustain star)

After burnout, the star can do any of a few things, depending upon its mass:

• supernova -> black hole (only the heaviest stars)
• supernova -> neutron star (stars heavier than our Sun, but not heavy enough for black hole)
• collapse to white dwarf and gradual cool down to brown dwarf (our Sun and most other stars)

Our Sun has been in the main sequence phase for about 5 billion years and will continue to be for another 5 billion years.

For more information, check out the "Imagine the Universe!" site.

Dr. Eric Christian

The answers to both your questions are found in a famous plot, the Hertzsprung-Russell (H-R) diagram. This diagram shows the relationship of a star's temperature to its brightness (or absolute magnitude). In general, brighter stars are hotter; cooler stars are dimmer. The vast majority of stars follow this pattern and are called "main sequence". Stars that don't follow this pattern lie above or below the main sequence stars in the H-R diagram. These are either bright, low temperature stars (so called giants and super-giants), or are dim but hot (white dwarfs).

For more information check out any college astronomy text book.

Dr. Louis Barbier and Beth Barbier

The subject of Cepheid variables is covered very well in our sister site, Imagine the Universe!, at:

• What is a Cepheid Variable?
• Cepheid Variables as Cosmic Yardsticks
• Measuring Distances with Astronomy

and at the Electronic Universe Project.

If you have any more specific questions after this, it would be best to ask them of the Imagine the Universe! team, as this is not our area of specialty.

Beth Barbier
(August 2000)

The effect you're seeing is almost certainly due to refraction in the atmosphere. The probable reason that only certain stars seem to change color is that it requires a certain light level before the human eye can distinguish colors (lower than that, you only see a grayscale). So only the brightest stars (Arcturus is the fourth brightest, fifth including the Sun) will exhibit this type of sparkling. Dimmer stars only twinkle (move around and change brightness).

Dr. Eric Christian

There are two processes involved here. One is the "intrinsic" color of the star, which is the color someone would see if they were not moving towards or away from the star. That color is completely determined by the temperature of the star. Blue and white stars are hotter than our Sun, red and orange stars are cooler. Each star actually emits a broad range of colors, but the peak emission (the color that has the most light) is what determines the star's color.

An observer sees all of the colors that the star emits shifted either towards the red or the blue depending upon whether the observer is moving away or towards the star. An intrinsically red star moving very quickly towards the observer "would" appear blue, and an intrinsically blue star away very fast "would" appear red. Most stars are not moving quickly enough for them to shift from red to blue or vice versa and we observe them at nearly their intrinsic color. But because of narrow "emission lines" that stars give off (very specific colors from one element), scientists can measure small red and blue shifts even when its overall color doesn't change.

Dr. Eric Christian
(March 2003)

Unfortunately, your question is beyond our area of expertise or interest. But the source for this story is the Harvard-Smithsonian Center for Astrophysics, and you can read their press release. There is contact information at the bottom of the page.

Beth Barbier
(February 2005)

The important things to know about what makes a star implode are:

1. Stars have a LOT of mass. Our Sun has more than 300,000 times the mass of the Earth, and it's only an average star. There are stars that are a lot heavier.
2. Stars have a pretty low density, especially when they are in their red giant phase (which is where they are at when they implode). They are so large because they are so hot, and the pressure from all the heat inflates them. The average density of a red giant is much less than one thousandth of the density of water.

To get a stellar implosion (leading to a supernova and then either a black hole or neutron star), the fuel at the center of the star gets used up, and the core cools rapidly. The pressure that has been holding up the outer layer drops, and the outer shells start falling toward the center. The gas has a long way to fall (the radius of a red giant can be as large as the distance from the Earth to the Sun) and builds up a tremendous speed. It all meets at the center, at which point most of the mass rebounds into a supernova explosion. About 20% of the mass gets compacted at the center and either forms a neutron star or (if the original star was really large) a black hole.

You can get more information on black holes and the like at the Imagine the Universe! site.

Dr. Eric Christian

Star clusters are self-gravitating groups of stars. They are spherical because there are not enough interactions to average out the angular momentums of the individual stars, which is what you need to do to form a disk. The distance between stars is large compared to the size of the stars. At the center, the stellar density may get high enough to create a massive object at the center and a disk of destroyed stars around it, but the cluster will still be mostly spherical. Indeed, most clusters don't have enough mass to really remain together, and will, over time, diffuse away.

Dr. Eric Christian

Yes, stars rotate. Conservation of angular momentum says that any spinning of the dust cloud that formed the solar system will remain, and since most of the matter in the solar system is in the Sun, the Sun will be spinning. It will even be spinning faster than the original dust cloud for the same reason that a skater spins faster by bringing in his/her arms. The lower the "moment of inertia", the faster the spin rate.

The Sun rotates about every 25 days at the equator and about every 36 days near the poles; it is more like a fluid than a solid body, so unlike the Earth, it doesn't have to have the same period. You can (taking the proper eye precautions), see the sunspots move across the face of the Sun. As for the direction, the Sun is spinning in the same direction as the planets are revolving about the Sun, the same direction the original cloud was spinning. There will be some tendency for all stars in the Milky Way to spin in the direction the Milky Way is revolving, but on an individual basis, the stars will rotate in somewhat random directions.

Dr. Eric Christian

There are a number of Star Catalogs on the world wide web, but most of them are meant for scientists and are in much more detail than you need. The closest star to the Sun, Alpha Centauri, is 4.27 light years away. The brightest star, Sirius, is 8.64 light years away.

Dr. Eric Christian

The parallax method of measuring star distances doesn't "require" observations of a star made 6 months apart, but observations made 6 months apart are best. This is because you want to make parallax measurements from two points that are as far apart as possible. Six months after your first observations, the Earth is on the other side of the Sun, 186 million miles away from where you started. This is the longest "baseline" you can get from the Earth.

Dr. Eric Christian

It does seem a bit unbelievable, but the important thing to remember is that stars give off a LOT of light. So, even though the closest star is 66,000 times farther from us than the Sun, and the amount of light that reaches us is (66,000 * 66,000) times fainter than the Sun, it is still enough to see. It is also 66,000 times smaller, and so looks just like a point. It is possible to measure the light coming from the Sun and the stars, and it really is all consistent.

Dr. Eric Christian
(March 2000)

I can find no star named "Tulare". There is a Tulare, California, however. I also don't know of a web site that lists stellar names. My suggestion is that you get a basic amateur astronomer's book, such as the Peterson's Guide to Stars and Planets.

Dr. Eric Christian

There are a few commercial companies that pretend to name stars after people for a fee. These names are only valid in their records and your own mind. These companies have no relationship with the International Astronomical Union (IAU) which does name stars, comets, asteroids, etc. Stars typically have historic names or catalog names, and are not named after people. For more information on how stars are named, visit the IAU's "Buying Star Names" web page.

Dr. Eric Christian and Beth Barbier