Patterns Among Stars

(Hertzsprung – Russell) plots stars according to: An H-R diagram (Hertzsprung – Russell) plots stars according to: Luminosity (vertical axis) Surface temperature (horizontal axis)

When you plot many stars this way, patterns start to emerge This is going to be a sort of road map of the lives of stars

Which is more luminous, a giant star or a white dwarf? A) A giant star B) A white dwarf C) Their luminosities are the same D) Cannot answer without more information

Which has a hotter surface, a giant star or a white dwarf? A) A giant star B) A white dwarf C) Their surface temperatures are the same D) Cannot answer without more information

(Don’t worry about Ia vs Ib) A complete description of a star would include not only its spectral class, but also its “luminosity class” Our Sun is a G2 V star (Don’t worry about Ia vs Ib)

Surface temperature and luminosity are related by radius: So radius can be determined for any star from luminosity and temperature

Which star would be the most luminous? A) R = 0.01 RSun T = 30,000 K B) R = 1 RSun T = 10,000 K C) R = 100 RSun T = 3000 K D) They’re all the same luminosity

large blue stars There are MANY more than for two reasons: small reddish stars than large blue stars for two reasons: Small stars live longer More small stars form By simple numbers of stars, our Sun is larger than average!

How long a star can fuse hydrogen (“Lifetime”) depends on the mass of the star: The more massive the star, the faster it uses up its fuel! 10 MSun  10,000 LSun Lives only 1/1000 as long as the Sun  10,000,000 yrs 0.3 MSun  0.01 LSun Lives 30 times longer than the Sun  300,000,000,000 yrs

The life stages of a star… old folks home The life stages of a star… More details later! productive life birth graveyard

Some stars change over short periods of time  “variable stars” A layer of ionized Helium at just the right depth is opaque. Heat can’t get out, causing the upper layers to expand. It expands past the equilibrium point, but eventually the density gets low enough for the heat to escape. The outer layers cool and contract again. It contracts past the equilibrium, making the He+ layer opaque again, trapping heat and starting the cycle over.

A typical Cepheid variable light curve Cepheid variable stars are bright and their period is closely related to their luminosity, making them a great distance measuring tool!

The Pleiades or Seven Sisters Star clusters Stars in a cluster: Are about the same distance from us Formed at about the same time Globular cluster M80 Open cluster The Pleiades or Seven Sisters Found in the halo Up to 1,000,000 stars or more 60 – 150 ly across Found in the disk Up to 3 or 4 thousand stars ~ 30 ly across

What would happen if we plotted all the stars in a cluster on an H-R diagram?

What would happen if we plotted all the stars in a cluster on an H-R diagram? They lie along the main sequence only up to a certain point “Main sequence turnoff point” The lifetime of stars at this point is the age of the cluster! This allows us to compare the ages of various clusters…

Comparing various cluster ages Open clusters are rarely more than 5 billion years old, usually much younger. Globular clusters can be much older…

Which cluster is oldest? A) h + c Persei B) Pleiades C) Hyades D) NGC 188

Globular clusters (this is M4) are found to be up to 13 billion years old! These clusters started forming before the universe was 1 billion years old

Astro-Cash Cab! Kaitlin Hoffman Jake Heaton Sabra Ross Brenna Hunter Mason Harvey

1) What are the axes on the HR diagram? a) Mass, Radius c) Brightness, Distance b) Temperature, Luminosity d) Mass, Temperature

2) What do we call this group of stars?

3) Which main sequence star has a higher luminosity?

4) Which star will run out of hydrogen fuel first?