Stellar Mass A star’s position on the main sequence is determined by mass. Mass and composition are set at a star’s birth.

We use spectroscopy to find the composition. (Which is the same for most stars).

Mass is found by the star’s gravitational influence on other bodies in space.

Fortunately most stars are in multiple star systems, so we are able to find the mass.

The majority of all stars are in binary systems; they are called binary stars. These come in different types. Visual binaries are binaries where two stars can be seen.

In spectroscopic binaries the Doppler shift of each star’s spectra can be seen. In double line spectra two sets of spectra are seen to shift. In single line spectra one set is too faint to be seen, but we still see the Doppler shift of the other star.

Eclipsing binaries - the starlight decreases as one star passes in front of the other. The variation in light is called the light curve.

Stars that are side by side as viewed from Earth may not be binaries. Optical doubles are NOT BINARIES. They just appear close together as viewed from Earth.

What can we learn from observing binary systems? the orbital period of the system (from hours to centuries), orbital velocities (from the Doppler shift), orbital size (if we know distance from Earth), the combined mass of the stars, and the ratio of the stellar masses.

The ratio of stellar masses and the combined mass allow the mass of each star to be determined.

More massive stars are usually hotter, larger, and more luminous. Less massive stars are usually cooler, smaller, and less luminous.

The radius of a star rises in proportion to the mass, luminosity increases as the cube of the mass. A two solar mass star is two times the Sun’s radius, but it has eight times the Sun’s luminosity.

Massive stars “die” faster than less massive stars. Most O and B stars are less than 20 million years old. Most K and M stars are at least 10 billion years old and should last another trillion years.

Clusters are groups of stars that are all the same distance from Earth. Their composition and age are probably the same, so the only distinguishing factor is mass. These are like natural labs for stellar studies.

Open clusters, like the Pleides, are at most 20 million years old. They are composed of a few tens to a few hundred stars and are a few parsecs across. It has O and B stars, therefore it is a young cluster. HR diagrams constructed for open clusters form a normal main sequence diagram.

Globular clusters are more tightly knit, are roughly spherical, and are composed of 10,000’s to millions of stars. They are about 50 parsecs across. O and B stars are absent indicating that globular clusters are older than open clusters, at least 10 billion years old. The HR diagram lacks O and B stars.

Hydrogen to Canis Majoris

Star Scale Montage (outdated)

Biggest Star Known (Canis Majoris)

Sometimes stars swell to incredible sizes as seen in the next slide:

Love ya, Jordan!!