10 October, 2001

Here are the answers to selected questions submitted between September 9 and October 2, 2001. Questions may have been edited for clarity or brevity. Click on a link to move directly to the answer.

1. How is it possible to determine the mass of a distant star, and can the mass of the sun be determined in the same way?

The mass of the sun is determined by using Newton's laws of gravity. If we know how far from the sun a planet (say the Earth) is, and we know how long it takes that planet to orbit the sun (one year for the Earth), we can calculate the mass of the sun. This same method is used to calculate the mass of the Earth, using the moon instead of another planet.

The same methods can be used to determine the masses of stars in a binary star system, where two stars orbit each other. If we can measure the distance to the star and map out the orbits of the stars on the sky, then we can calculate the mass of each component star. This is useful for nearby stars that orbit each other in, say, a hundred years or less (so that the orbits can be well-mapped). The star Sirius is an example of this.

For most stars, the measurement of their mass is determined from a computer model of the star. As a star's mass changes, its absolute brightness, its size, its temperature, its color, and even its detailed spectrum change. Computer models tell us how these change with a star's mass. So when we see a random star in the sky, we can compare what we see to what the computer models predict. The model that is closest to the observed star's properties gives us the mass of the star!

These models are constantly being improved and compared to stars whose masses are known by other means, such as the sun and binary stars. The models seem to work very well for most stars, and the models work extremely well for the sun, so we are confident that our measure of other star's masses are pretty close to correct.

For the record, the sun's mass is approximately 2,000,000,000,000,000,000,000,000,000,000 kilograms, or 333,000 times the mass of the Earth.

2. I was wondering if stars ever twinkle different colors. Last night, I saw two stars that sparkled red, white, and blue.

Stars twinkle because the Earth's atmosphere bends the star's light on its way down to your eye. Sometimes slightly more of the star's light reaches your eye, and sometimes slightly less, which makes the stars twinkle.

You asked about stars that quickly change colors as you look at them. This is because the effect of the Earth's atmosphere on the star light also depends on the wavelength of the light. Sometimes the red light and the blue light from the star don't twinkle in the same way. If the atmosphere bends most of the star's blue light so that more of it reaches your eye, while the amount of red light stays the same, then the star will appear more blue for a second. The same thing can happen with the red light, making the star appear slightly redder. This should be the most noticeable when stars are near the horizon, and therefore passing through more air than when they are high in the sky.

There is a rather striking animation demonstrating the effect of the atmosphere on a star's light in the Astronomy Picture of the Day Archives. If you imagine the star's red light and blue light independently jumping around as shown in the animation, you can see why a star may appear to change colors as it twinkles.

3. I've read that the early universe became transparent when the hydrogen gas became ionized. That seems opposite since I learned that the reason the photosphere of the sun is opaque because it is ionized, and hydrogen gas is transparent when it is not ionized such as in the atmosphere, or in glass beakers and such.

This question refers to the 'epoch of reionization', a period in the very early universe which astronomers have long thought existed, and have recently found convincing evidence:

As the very early universe was expanding and cooling, electrons and protons- which had been free particles up to this point- came together and formed neutral hydrogen gas, such as what you would see in a beaker of hydrogen gas. Neutral hydrogen gas is opaque to high energy photons- a photon with enough energy to ionized hydrogen (i.e.. remove the electron from a hydrogen atom)- because if it encounters a hydrogen atom, the photon is absorbed.

As the universe continued to cool, clouds of hydrogen gas collapsed to form the first generation of stars. As these stars formed they emitted high energy radiation through the processes of nuclear reactions. These stars emitted enough high energy photons to completely ionize the neutral hydrogen gas surrounding them, so that no neutral hydrogen was left. Thus additional photons were not 'captured' by hydrogen and could move about freely- the universe had become transparent. This is called the 'epoch of reionization'.

Before the epoch of reionization, the universe was opaque to high energy photons (e.g. UV or X-ray photons). Because of the expansion of the universe, this light has been stretched to visible light. That is why astronomers call this early time the "dark ages," as the optical light we collect on earth, which was ultraviolet light in the early universe, was absorbed. However, to lower energy photons such as optical or infrared, the universe was transparent both before and after this epoch.

For further reading, check out the an intermediate-level review on the epoch of reionization

4. How often has Earth's magnetic field reversed itself?

The Earth's magnetic field is not as constant as it appears to us. On timescales much longer than one human's life the magnetic field of Earth undergoes polarity reversals. Geologists studying Earth's paleomagnetic magnetic record have found ample evidence for such reversals.

During the formation of certain types of rocks small magnetic particles are "frozen" in, with their magnetic dipole pointing in the direction of Earth's magnetic field at the time. Magnetic crystals in lava and iron compounds in sedimentary rocks are two important types studied. Geologists use sophisticated equipment to detect these particles and the direction their dipoles are pointing in. By comparing data from different regions of the Earth and from different formation periods they can get a glimpse of the history of Earth's magnetic field. Unfortunately it is impossible to tell exactly how many times the Earth's magnetic field has reversed itself. Apparently Earth's magnetic field reversals are not periodic, unlike the Sun's, which have a period of roughly 11 years. Paleomagnetic studies indicate that the time between reversals can be as short as 100,000 years but as long as 25 million years.

In addition to geological field work some scientists have taken a theoretical path towards understanding Earth's magnetic field. Gary Glatzmeier here at UCSC's Earth and Marine Science Department was the first person to create a self-consistent three-dimensional computer model of the Earth's magnetic field. His model, too, exhibited polarity reversals on a timescale similar to the one observed on Earth.

For more information please see a Scientific American article on this very topic.

5. Why do comets have two tails instead of one like a dog? (This question was submitted by a young school-age student, and is an excellent question for any age!)

Comets are sometimes called "dirty snowballs." They are made of ice and dust all mixed together. When a comet comes close to the sun, some of the ice melts, and the dust mixed in gets blown into space. The melted ice forms one tail, called the ion tail. The dust forms another tail, called the dust tail.

The two tails of a comet point in different directions. The ion tail points straight away from the sun, because the solar wind (which originates from the sun) carries the gases away. The dust tail follows the orbit of the comet (pushed outward slightly by sunlight), and so is curved.

Below is a picture of Comet Hale-Bopp taken by astronomer Richard Wainscoat from the University of Hawaii (click on the image to be taken to a page with more pictures). The ion tail is blue (due to the type of gas in the tail), and the dust tail is curved and white. (Photograph © 1997 Richard Wainscoat.)

A comet: two tails

A dog: one tail

To read more about comet tails, please see a tale on comet tails.

Thanks to Marla Geha, Mike Kuhlen, and Greg Novak for helping to answer these questions!