Astronomy Notes

Optical Doubles

• 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 Alcor.
• 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.

Binary Stars

• 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

• 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.

Spectroscopic Binaries

• 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.

Eclipsing Binaries

• 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 brighter star.

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.

Contact Binaries

• 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.

Novae

• 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.

Epsilon Aurigae

• One weird binary is the star which is the goat that Auriga (the Charioteer) is holding (epsilon Aurigae).
• 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 companions.
• 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

• 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.

Open Clusters

• 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 galactic clusters.

Globular Clusters

• 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.

Evolution

• 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.