RR Lyrae variable

Some stars change their light.

They grow bright and then dim. This happens many times. These stars are very old. They are often in groups called clusters.

These stars pulse like a heartbeat. They can pulse in less than one day. Sometimes they pulse every seven hours.

Scientists use them to find distances. They act like little lights in the sky.

These stars are smaller than our Sun. They are much more common than other pulsing stars. They help us learn about space.

Some stars change their brightness over time. We call these variable stars.

RR Lyrae stars are a special type of variable star. They are named after a bright star called RR Lyrae. These stars pulse like a heartbeat. They can change brightness in less than one day. Some pulse every seven hours.

These stars are very old. They are often found in globular clusters. These are large groups of stars. Most RR Lyrae stars are in these clusters.

Scientists use them as standard candles. A standard candle is a tool to measure distance. Because we know how bright these stars are, we can find how far away they are. This helps us map our galaxy.

RR Lyrae stars are smaller than our Sun. They have about half the mass of the Sun. They are also metal-poor. This means they have fewer heavy elements. Many of these stars are in the halo of the Milky Way. The halo is the outer part of our galaxy.

RR Lyrae stars are a special kind of variable star. This means they change their brightness over time. They are very old stars that belong to a group called Population II. You can often find them in globular clusters. These are huge, round groups of stars. Most of these stars are found in the halo or the thick disk of our galaxy.

These stars work by pulsing, much like a heartbeat. This pulsing happens because of something called the kappa mechanism. Inside the star, the opacity of ionised helium changes with its temperature. This change in how light passes through the gas causes the star to swell and shrink. This makes the star look brighter and then dimmer. Most RR Lyrae stars have a very short period. They might pulse in less than one day. Some even pulse every seven hours.

People have been studying these stars for a long time. A star named RR Lyrae is the prototype for this whole group. Williamina Fleming discovered it before the year 1899. Later, E. C. Pickering reported on it in 1900. In the mid-1890s, many of these stars were being found in clusters. J. Kapteyn found a star called U Leporis in 1890. This was likely the first one found outside of a star cluster.

There are many interesting facts about these stars. They have about half the mass of our Sun. They are much more common than Cepheid variables. In the 1980s, about 1,900 were known in globular clusters. Some estimates say there are 85,000 in the Milky Way. Astronomers divide them into three main types. The RRab type is the most common at 91 percent. The RRc type makes up 9 percent. The rare RRd type is a double-mode pulsator.

Scientists use RR Lyrae stars as standard candles. A standard candle is a tool used to measure distance in space. Because we know how bright they are, we can calculate how far away they are. This helps us build a cosmic distance ladder. The Hubble Space Telescope has even found them in the Andromeda Galaxy. The Gaia mission has mapped 140,784 of these stars. This helps us understand the shape and age of our universe.

RR Lyrae variables are a specific class of periodic variable stars. These stars change their brightness at regular intervals. They are old, low-mass stars known as Population II stars. They are often found in globular clusters, which are dense groups of stars. They also appear in the halo and the thick disk of the Milky Way.

Because they are old, they help astronomers trace the history of our galaxy.

The way these stars pulse is quite fascinating. They undergo a process called the kappa mechanism. This happens when the opacity of ionised helium changes based on its temperature. Opacity describes how much a material blocks light. As the temperature changes, the helium gas becomes more or less transparent. This causes the star to swell and shrink in a rhythmic cycle. This pulsing makes the star appear to change brightness to observers on Earth.

Astronomers classify RR Lyrae stars into three main types. These types are based on the shape of their brightness curves, which were defined by S.I. Bailey. The most common type is the RRab variable. These make up 91% of all observed RR Lyrae stars and show steep rises in brightness. The RRc type makes up 9% of the group. These have shorter periods and a more sinusoidal, or wave-like, variation. Finally, the RRd type is quite rare. They make up between less than 1% and 30% of a system and are double-mode pulsators.

History shows how our understanding of these stars has grown. The prototype star, RR Lyrae, was discovered before 1899 by Williamina Fleming. In 1900, E. C. Pickering reported that it was indistinguishable from cluster-type variables. In the mid-1890s, many cluster-type variables were identified by Pickering. J. Kapteyn discovered U Leporis in 1890, which was likely the first RR Lyrae found outside a cluster. Between 1915 and the 1930s, scientists accepted them as a distinct class from Cepheid variables. This was due to their shorter periods, different chemical makeup, and different locations in the galaxy.

These stars serve a vital role as "standard candles." A standard candle is an object with a known brightness used to measure distances in space. By knowing how bright an RR Lyrae star actually is, scientists can calculate how far away it is. This helps build the cosmic distance ladder. In the 1980s, about 1,900 RR Lyrae stars were known in globular clusters. Some estimates suggest there are 85,000 in the Milky Way. They are much more common than Cepheid variables, though they are less luminous. Their average absolute magnitude is about +0.75, making them 40 to 50 times brighter than the Sun.

There are several technical challenges when using these stars. One issue is called blending. This happens in dense areas like the cores of globular clusters. In low-resolution observations, multiple stars may look like a single target. If this happens, the measured brightness is erroneously too high. This can lead to wrong distance calculations. This uncertainty can affect estimates of the Hubble constant and the age of the Universe.

Modern technology has greatly expanded our knowledge of RR Lyrae variables. The Hubble Space Telescope found them in the Andromeda Galaxy's halo. The Gaia mission mapped 140,784 RR Lyrae stars. Interestingly, 50,220 of these were not previously known to be variable. The PanSTARRS1 survey identified about 45,000 stars, covering three-quarters of the sky. The Kepler space telescope also observed 37 known variables in its field. These observations help us understand the chemistry and quantum mechanics of older stars.