Part 13: Multiple Stars and Star Clusters
John P. Pratt
- An optical double is merely two stars that happen to be nearly on the same line of sight.
- The two stars are not physically associated in any way.
- An example is Mizar (the middle star of the three in the Big Dipper's Handle) and
- Mizar and Alcor used to be an Arab eye test, which is strange because it's a very easy test now.
- Optical doubles are not important in astronomy, so no more will be said about them.
- The word binary is used for stars which are in orbit around each other.
- They represent the first discovery that gravity is at work outside of our solar system.
- They are more common than single stars--over 2/3 of stars are in binary or multiple systems.
- They provide the best way to determine the mass of stars, by using Kepler's Laws.
- They are discovered in many ways, which leads to many different classifications of binaries.
- Visual Binaries can be seen in a telescope to be two separate stars.
- Even large telescopes are limited to about 1" of arc separation by air quality, the same as a
telescope with only a one foot diameter mirror.
- There are many beautiful visual binary stars, with stars of very different colors.
- The brighter component is labeled A, and the dimmer B, as in Sirius A.
- A nice double star in binoculars is Epsilon Lyrae, which is within one degree of Vega.
- Mizar (at the middle of the Big Dipper's handle) is a visual binary in small telescopes.
- One beautiful pair is the blue and yellow Albireo, the star which is the bottom star of the
the Northern Cross.
- Some binaries are too close to see visually but can be discovered by red shifts in their spectra.
- That is, one or both of the stars can be seen coming toward us or moving away from us.
- To have such a fast orbital motion always means that they are too close to be a visual binary.
- Both of the visual components of Mizar are also spectroscopic doubles.
Algol, the head of the Medusa which Perseus holds, it the most famous eclipsing binary.
- Eclipsing Binaries pass in front of each other, which dims the light coming from them.
- From the light curve one can deduce their relative sizes and positions.
- Most are also spectroscopic binaries, so we can get a lot of information about them.
- The primary is a hot blue star with M = 5, secondary is an orange giant with M =1.
- It dims by a full magnitude in only 4 hours, every 2.9 days, when the giant star eclipses the
Evolution of Binaries
- A figure "8" called the Roche surface can be drawn around the two stars.
- Each half of the figure is called a Roche lobe.
- The point where the two lobes meet is the point of equal gravity between the stars.
- When a star expands as it evolves, it can fill its Roche lobe.
- When it does, matter streams through the equal gravity point onto the other star like sand
through an hourglass.
- Sometimes over half of the expanding star can transfer onto the other stars.
- That what apparently happened on Algol: The orange giant was originally the more massive.
- If both stars fill their Roche lobes, it is called a contact binary.
- The periods of orbital revolution are usually less than 2 days.
- Their evolution can be complicated, and they can be very unusual stars.
- A nova is a star that flares up in much increased brightness.
- The explosion is not nearly so violent as in a supernova.
- It is believed that all novae are binary stars, and that one star is expanding through its Roche
lobe, transferring hydrogen onto a white dwarf.
- That hydrogen is greatly compressed and can explode with nuclear reactions.
- If the matter falls on the white dwarf fast enough to exceed the Chandrasekhar limit, the
white dwarf could also explode as a supernova.
Binaries with two jet streams
- Some binaries have huge jets of gas shooting out of both sides, with velocities of 25% of the
speed of light.
- These give off X-rays and gamma rays.
- The star producing the two jets is orbiting around another star.
The search for Planets around other stars
- One weird binary is the star which is the goat that Auriga (the Charioteer) is holding (epsilon
- The primary is a yellow-white super giant, as large as the orbit of Mars.
- Every 27 years it is eclipsed by a huge disk of dust for 2 years, absorbing half its light.
- It now appears that at the center of the disk is a close binary that stirs up the dust dumped
onto it by the supergiant.
- Astronomers have hunted for years for planets around other stars.
- They have tried to find them mostly by looking for oscillations of a star around invisible
- In the last few years, it is believed that several have been found.
There are three basic kinds of clusters, based mostly on how tightly clustered the stars are.
- An association of stars is so loosely packed that it is not even held together gravitationally.
- An open cluster is a moderately close-knit, irregularly shaped group of 100-1,000 stars.
- A globular cluster contains about 100,000 stars and is distinctly spherical shaped.
- Associations are being ripped apart by galactic tidal forces, even as Saturn's rings are particles that
are separated by Saturn's tidal forces.
- They often have an open cluster at their center, which is still gravitationally intact.
- There are two kinds of associations, formed of rather different types of stars.
- O associations are composed of O and B stars (huge blue stars, often in gas).
- T associations are composed of T Tauri stars (giant red stars with dust clouds)
- An example of an association is the head of Perseus; it is a beautiful field in binoculars.
- The four stars at the center of the Orion Nebula (the Trapezium) might be a tiny association.
- The nearest open cluster is the Ursa Major cluster, which include all but the end stars of
the Big Dipper, being about 70 l.y. away.
- Some have suggested that our sun may be part of the Ursa Major cluster.
- The Hyades and Pleiades (Seven Sisters) and Beehive are some of the
next closest open clusters.
- Open clusters tend to be found in the plane of our galaxy; hence they are sometimes called
- Globular clusters are tightly gravitationally bound, being nearly spherical shaped.
- They typically contain many stars evolving off the lower main sequence to red giants.
- They are found spherically distributed around our galaxy, not in the plane.
- They look like little spherical fuzzy patches in small telescopes, an 8 inch can see stars.
- One of the brightest examples is the Hercules Cluster on an edge of the trapezoid in Hercules.
- Many globular clusters are strong X-ray sources, perhaps from many neutron stars.
Measuring Distances to Clusters
- Parallax only works for the very nearest clusters, like the Hyades.
- Estimating the luminosity from the H-R diagram works, especially fitting the main sequence.
- Measuring the period of Cepheid variables gives luminosities for some globular clusters.
- Measuring the diameter of more distant globular clusters gives an estimates.
- One has to allow for the interstellar reddening from dust particles.
- About half of open clusters have so many massive blue stars that they appear to be young.
- Globular clusters often don't have any bright blue stars and are thought to be very old.
- All clusters in the plane of our galaxy are being disrupted; almost no globulars are known there.