Properties of Main Sequence Stars Masses Luminosities Lifetimes Distribution

How do we know the masses of stars? By studying binary stars! “The mass of any central object can be determined if it has an observable orbiting satellite.” Center of mass

Stars have a fairly narrow range of masses.

Sun-like stars are 1 solar mass

Stars have a fairly narrow range of masses. Sun-like stars are 1 solar mass O & B stars are a few 10’s of solar masses

Stars have a fairly narrow range of masses. O & B stars are a few 10’s of solar masses M stars are a few 10’ths of a solar mass Sun-like stars are 1 solar mass

How do we know the luminosity of stars? 1.From direct measurement of a star’s distance through stellar parallax and its apparent magnitude we can calculate the absolute magnitude: M = m - 5log(D) From theoretical nuclear fusion calculations the luminosity can be modeled as a function of spectral type with reasonable accuracy. (After all we make the best nuclear bombs!)

Stars have a HUGE range of luminosities.

Sun-like stars are 1 solar luminosity

Stars have a HUGE range of luminosities. O & B stars are 10,000 solar luminosities Sun-like stars are 1 solar luminosity

Stars have a HUGE range of luminosities. O & B stars are 10,000 solar luminosities M stars are a few 10,000’ths of a solar luminosity Sun-like stars are 1 solar luminosity

How do we know stellar lifetimes? 1.Again, theoretical nuclear modeling yields estimates of main sequence lifetimes. 2.However, a simple estimate can be made based upon the mass of the star and its luminosity as shown below. Spectral Type Mass M  Luminosity L  Lifetime in Solar Lifetimes O stars3030,00030/30,000 =.001 M stars /.0003 = 1,000

Stars also have a HUGE range of lifetimes.

10 Billion Years for Sun-like stars

Stars also have a HUGE range of lifetimes. 10 Billion Years for Sun-like stars A few Million Years for O stars

Stars also have a HUGE range of lifetimes. 10 Billion Years for Sun-like stars A few Million Years for O stars A few Trillion Years for M stars

Stars also have a HUGE range of lifetimes. 10 Billion Years for Sun-like stars A few Million Years for O stars A few 10’s of Trillion Years for M stars The Age of the Universe is now considered to be 13.8 Billion years ± 1%

Stars also have a HUGE range of lifetimes. 10 Billion Years for Sun-like stars A few Million Years for O stars A few 10’s of Trillion Years for M stars The Age of the Universe is now considered to be 13.8 Billion years ± 1% None of the first O through G stars exist anymore.

Stars also have a HUGE range of lifetimes. 10 Billion Years for Sun-like stars A few Million Years for O stars A few 10’s of Trillion Years for M stars The Age of the Universe is now considered to be 13.8 Billion years ± 1% Every M star ever created is still a main sequence M star.

The HUGE range in stellar lifetimes is reflected in the distribution of stars in the Milky Way Galaxy. Long lived K & M stars are most abundant. Short lived O & B stars are very rare. Other very luminous stars (Giants) will also be rare due to their short lifetimes. White dwarf stellar remnants have VERY long cooling times but are very low luminosity. They may represent the most common object in the galaxy but are undercounted.

Note the high abundance of M dwarfs.

Note the low abundance of O & B stars.

There are also very few giant stars.

There are probably many more white dwarf stellar remnants that are too dim to see.

Only 7% of the stars in the Milky Way galaxy are Sun- like and half of them are in multiple star systems. Thus, star and planetary systems like our own represent less than 3% of all the stars in the galaxy.

The most common stars in the Milky Way Galaxy are of such low luminosity that they are difficult to see even when they are right “next door”. These stars are utterly invisible in distant galaxies.

The very rare short lived stars are the most luminous and are the easiest seen in the night sky and in other distant galaxies

This Spiral Galaxy resembles our Milky Way Galaxy

This is a more distant galaxy of a different type. Hot O and B stars dominate the luminosity in the Spiral Arms There are very many low luminosity K and M stars in the apparently empty space between the spiral arms.

In the following M101 galaxy images you will see the galaxy through a red filter and then through a blue filter. (Repeats once) Notice that the galaxy is dimmer and less defined in red light and brighter and sharper in blue light. Only the most luminous stars show up distinctly in distance galaxies. The most common stars in the galaxy are individually invisible.

Red Filter Spiral Galaxy M101

Blue Filter Spiral Galaxy M101

Red Filter Spiral Galaxy M101

Blue Filter Spiral Galaxy M101

75% of the Milky Way’s luminosity arise from the rarest stars. K & M stars account for ¾’s of the stars in the galaxy but contribute less than 5% of its luminosity.

The properties of O and M Main Sequence Stars O Stars Mass  25 M  Radius  6 R  Temperature  30,000K Luminosity  80,000 L  M  -6 Main Sequence Lifetime  3 Million years Mass  0.1 M  Radius  0.1 R  Temperature  3,000K Luminosity  L  M  15 Main Sequence Lifetime  10 Trillion years M Stars For the Test