Measuring the Milky

One way to gauge the size of the Milky Way would be to look at other, similar galaxies, such as Andromeda. If we know how far away such a galaxy is, and we can measure its angular size on the sky, we can calculate how big it is.

A variable star is a star that periodically changes in brightness. A cataclysmic variable is a star, such as a nova, that changes in brightness suddenly and dramatically, as a result of interaction with a binary companion star. An intrinsic variable changes brightness because of rapid changes in its diameter. Pulsating variables are intrinsic variables that vary in brightness in a fixed period or span of time.

But determining the distance of Andromeda and the other galaxies can be a serious challenge. Parallax as a distance indicator is out, since we know that the apparent angular shift is equal to 1 divided by the distance in parsecs. Stars farther than 100 pc are too far to use the parallax method, and those stars are still in our own Galaxy. Half a microarcsecond resolution (0.0000005") would be required to see parallax of even the nearest galaxy, Andromeda.

One very good distance indicator was discovered in 1908 by Henrietta Swan Leavitt, working at the time under the direction of Edward Pickering at the Harvard College Observatory.

Over many centuries of star gazing, astronomers had noted many stars whose luminosity was variable— sometimes brighter, sometimes fainter. These variable stars fall into one of two broad types: cataclysmic variables and intrinsic variables. We have already discussed one type of cataclysmic variable star. Novae are stars that periodically change in luminosity (rate of energy output) suddenly and dramatically when they accrete material from a binary companion.

Another type of variable star is an intrinsic variable, whose variability is not caused by interaction with a binary companion, but by factors internal to the star. The subset of intrinsic variable stars important in distance calculations are pulsating variable stars. Cepheid variable stars and RR Lyrae stars are both pulsating variable stars.

Star Words

A variable star is a star that periodically changes in brightness. A cataclysmic variable is a star, such as a nova, that changes in brightness suddenly and dramatically, as a result of interaction with a binary companion star. An intrinsic variable changes brightness because of rapid changes in its diameter. Pulsating variables are intrinsic variables that vary in brightness in a fixed period or span of time.

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Astronomer's Notebook

The names of the class of variable stars, RR Lyrae and Cepheid, are derived from the names of the first stars of these types to be discovered. RR Lyra is a variable star (labeled RR) in the constellation Lyra; RR Lyrae is a genitive form of this name. The Cepheid class is named after Delta Cepheus, a variable star in the constellation Cepheus. Cepheid variables have longer periods (as a class) and are more luminous than RR Lyrae stars. RR Lyrae stars have periods of less than a day. Cepheid periods range upward of 50 days.

Close Encounter

Close Encounter

Henrietta Swan Leavitt was one of a number of talented astronomers on the staff of the Harvard College Observatory. She was the first to propose, in 1908, that the period of a certain type of intrinsic variable star (a Cepheid variable) was directly proportional to its luminosity.

She had observed a large number of variable stars in the Magellanic clouds (companions to our own Galaxy). The advantage of studying these clouds is that whatever the distance to them, the stars within them are all roughly at the same distance from the earth. What Leavitt noticed was that the brightest Cepheid variable stars in the Magellanic clouds always had the longest periods, and the faintest always had the shortest periods.

What her discovery meant was that astronomers could simply measure the period of a Cepheid variable star and its apparent brightness to derive its distance directly. What Leavitt accomplished was to greatly extend the astronomer's ruler from a few hundred light-years (using the parallax method) to tens of millions of light-years.

The pulsating variables vary in luminosity because of regular changes in their diameter. Why does this relationship exist? Stars are a little like ringing bells. When struck, large bells vibrate more slowly, producing lower tones. Tiny bells vibrate rapidly, producing higher tones.

A pulsating variable star is in a late evolutionary stage and has become unstable, its radius first shrinking and its surface heating. Then its radius expands and its surface cools. These changes produce measurable variations in the star's luminosity (because luminosity depends on surface area, and the star is shrinking and expanding).

The two types of pulsating variables are named after the first known star in each group. The RR Lyrae stars all have the similar average luminosity of about 100 times that of the sun. Cepheid luminosities range from 1,000 to 10,000 times the luminosity of the sun. Because both pulsating variables can be recognized by their pulsation pattern and average luminosity, they make convenient markers for determining distance. Because they are intrinsically more luminous, Cepheid variable stars are useful for measuring greater distances.

An observer simply locates such a star and measures its apparent brightness and period of variation. Then, using the star's known luminosity (from its period), she can calculate its distance.

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