Patterns Among Stars ––– Star Clusters

We have been talking about the HR diagram… Main Sequence stars are all fusing hydrogen into helium in their cores Note order: Small-to-large (mass and radius) Cool-to-hot RED DWARFS

There are MANY more small reddish stars than large blue stars for two reasons: 1.Small stars live longer 2.More small stars form By simple numbers of stars, our Sun is larger than average! Main Sequence stars

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 M Sun  10,000 L Sun Lives only 1/1000 as long as the Sun  10,000,000 yrs 0.3 M Sun  0.01 L Sun Lives 30 times longer than the Sun  300,000,000,000 yrs 1. Small stars live longer RED DWARFS

2. More small stars form Among the brightest stars we can see from Earth, many are very luminous. But is this representative of the population of stars as a whole?

2. More small stars form Looking at all stars within 12 ly, most of them are small. Spectral Types M & K are by far the most common.

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

Off the Main Sequence

The most common spectral type of star is A) B B) A C) F D) G E) M

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!

Star clusters Globular cluster M80 Open cluster The Pleiades or Seven Sisters Stars in a cluster: Are about the same distance from us Formed at about the same time 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 we see 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 +  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! Conner V Jasmyn Merlin Adrianna

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 size main sequence star is the most common? (Examples)

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