What is a variable star? Why does the brightness of this star change periodically?

Variable stars are stars whose brightness and electromagnetic radiation are unstable and constantly changing, accompanied by other physical changes.

When we look up at the stars in the night sky, we generally think that the brightness of these stars is relatively stable, burning their nuclear fuel at the rate of billions of years. Only in the last stage of life will the appearance of a star change, become a red giant, and finally end life.

But for many stars, change? Is part of their normal life. The European Space Agency showed a famous variable star, RS Puppis, in a spectacular way. Its brightness changes with time, which shows this magical change in the reflected light of the surrounding substances.

So what is a variable star? How does its brightness change? We can answer this question from three different angles, including historical angle, scientific angle and physical angle.

Historically,

Since ancient times, people have thought that the stars in the sky are a fixed point of light. Occasionally, catastrophic events like nova or supernova will happen, which will cause objects to brighten temporarily, but this is not common. There are only a few supernova events visible to the naked eye in human history.

1August 596, David? Fabricius discovered a star that suddenly brightened. At the end of 10, the brightened star completely disappears from sight. At first, he thought it was a new star. But strangely, this light spot appeared in the same position in 1609, and nova and supernova never appeared again in such a short time after dimming before; Therefore, the bright star discovered by Fabricius is not a new star at all, but the first star with variable brightness in human history, namely grasshopper variable star or Milla variable star.

But the scientific conditions at that time could not study such a rare astronomical phenomenon at all. This is not only rare, but also difficult to observe.

Therefore, variable stars are considered extremely rare in the universe at first, because after nearly two centuries of searching, people have discovered 10 such stars, but with the development of science and technology and astrophotography technology, variable stars become less mysterious and the number of discoveries has greatly increased.

1893, Henrietta? Levitt came to work at the Observatory of Harvard University and began to study variable stars. By 19 13, she has found about 1000 invariants in small magellanic cloud.

Through careful study, Levitt found that these variables have some special properties, such as: the variable with the highest average brightness has the longest luminosity change period, which means that it takes more time to complete a pulse period (from the darkest to the brightest, and then from the brightest to the darkest).

The cycle of the brightest variable is usually several months. When the brightness decreases, the change period will be shortened, and the darkest variable star can complete a pulse period in one day.

According to the above properties, Levitt found that there is an obvious relationship between the average brightness of Cepheid variable and the pulse period.

This correlation is what we often call the period-luminosity relationship today. This great discovery has brought immeasurable influence to the development of cosmology. Let's talk about the role and significance of knowing variable stars in science.

The role and significance of science

These variables investigated by Levitt are all in small magellanic cloud. The small Magellanic Cloud is about 7000 light years away from the Earth 199000 light years. Because of its small size, we think that all these variables are about the same distance from the earth. Different apparent brightness corresponds to different intrinsic brightness of variable stars, so we can calculate the relationship between intrinsic brightness and period of variable stars according to the relationship between distance and brightness.

Knowing this, the next time we find a variable star in other galaxies, we only need to measure the change period of the brightness of the variable star, and then we can know how big the variable star actually is, and then we can measure the how bright of the variable star visually, and then we can calculate how far it is from us according to the attenuation relationship between luminosity and distance.

We now call these objects that can measure the distance standard candlelight, that is to say, we only need to know that a candle has a how bright, and then measure that it looks like a how bright, so we can know how far the candle is from us.

Mastering the Zao Fu period-luminosity relationship, we have a standard candle. So we can use it to measure the distance between galaxies in the universe, Edwin? Hubble used variable stars to measure that the spiral nebula is actually an independent galaxy far away from us in the 1920s. Only then did I realize that the universe is not only the Milky Way, but also a large number of Sing Tao similar to the Milky Way.

There are many kinds of variable stars in the universe, with great differences in color and brightness. In addition to Cepheid variable discovered by Levitt, there are many variable stars with low quality and short period, such as RR variable in Lyra, red giant variable (such as Milla) and constantly beating white dwarf.

But the most important thing is that the periodic changes of these variable stars are easy to be observed in the universe, and there is a very obvious correlation between their periods and their absolute brightness, so variable stars are one of the most important parts of the cosmic distance ladder.

Of course, there is a better way for us to measure the distance at close range, that is, the parallax method that we first mastered, that is, the earth revolves around the sun once in a year, and the position of the star in the sky changes to determine the distance of the star, but this method is only applicable to stars with a distance of 1600 light years.

Therefore, in the range of 1600 light-years from the earth, we usually use parallax to measure, but in the farther distance, we use variable stars, and the measured distance has exceeded1000 light-years!

By observing how the brightness of these variable stars changes with time, that is, how long their change period is, and then identifying what kind of variable stars we observe, we can determine the distance from the celestial bodies hundreds of millions of light-years away from the Milky Way.

So, the last question is, why do the luminosity of these stars change? But also presents a stable periodicity,

Physical reasons

When the luminosity of a star changes, we generally think that the core nuclear fusion rate changes, and then these changes spread to the surface, resulting in periodic changes in the luminosity of the star. But this situation is basically impossible. First of all, even if the core fusion rate of a star fluctuates, it will take at least hundreds of thousands of years to affect the luminosity, because it takes at least about 654.38 million+years for a typical photon to propagate to the surface of the star.

In addition, in the life cycle of a star, the rate of core fusion will change, but this happens on a large time scale and will not fluctuate in the short term. In fact, the core fusion rate of various types of variable stars is stable, but not the same.

In order to explain the fluctuation of the variable star period, we need to look at the outermost layer of the star.

The outermost layer of a star is the photosphere. After leaving the photosphere, photons are permanently separated from stars, but the photosphere is a special place. For those stars that are not variable stars, the photosphere is relatively stable. That is to say, the radiation pressure and gravity that push the ions outward in the photosphere are balanced with each other, and the two forces cancel each other out. The sun is such an approximate situation, but it is not a perfect balance.

In the outermost layer of the sun, radiation pressure and gravity are also confronting each other. You push and I pull, causing matter to rise and fall between the balance of these two forces, so the outermost layer of any star will have the following cycle:

At a certain point, the radiation pressure becomes too large, which overcomes the gravity and causes the outer city of the star to expand.

When the matter in the outer city is far away from the center of the star, gravity will drop, but the thrust provided by radiation pressure will drop faster than gravity.

So after the outer city expands to a point, gravity overcomes the radiation pressure and pulls the matter back.

The gravitational pull-back process will cause the material in the outer layer of the star to contract inward.

Then, radiation pressure began to rise again. When it reaches a certain point, it starts to push the outer city of the star outward, and then it moves back and forth!

For the sun, its brightness variation intensity is about 0. 1%.

But for variable stars, their brightness and radius can change dramatically, such as more than 90%! The radius usually changes by millions of kilometers, and the temperature changes by thousands of degrees!

This is the history of variable star discovery, its application in cosmology and the reason for its luminosity change. In fact, for normal stars, if we observe them accurately enough, we will find that the brightness of each star will experience such periodic changes. Like many things in this universe, the only constant is change.