Star Cluster Trivia!

Open Clusters Hyades Pleiades

Globular Clusters Omega Centauri M15

What are star clusters?

Group of stars, bound together by mutual gravitational pull among all stars

Describe main differences between Open and Globular Clusters SizeShape

Size Open clusters have fewer stars than globular clusters Open clusters are more sparse, globulars have higher density or concentration of stars Shape globular clusters are very spherical and symmetric open clusters are more irregular

How many stars do they have? Open Clusters Globular Clusters

Open Clusters hundreds to thousands ~ Globular Clusters hundreds of thousands to millions ~100,000 to 1,000,000 How many stars do they have?

How many clusters are there in the Milky Way? Open:Globular:

Open: over 1100 known, may be more (up to 10,000) Globular: ~150, may be more There may be many more clusters than we know of so far - they can be hidden by clouds of gas and dust in the Galaxy!

How old are the clusters in the Milky Way? Open:Globular:

Open: less than a billion years old (typically a few hundred million years old) Globular: billion years old (much older than open clusters)

Open Clusters: Globular Clusters: 50 kiloparsecs Where do star clusters reside within the Milky Way?

Open Clusters: Disk or plane of the Galaxy Globular Clusters: Halo of the Galaxy Halo Globular Clusters Disk Bulge Galactic Nucleus Sun 50 kiloparsecs

How dense are clusters? We can measure the density in terms of Open Clusters: Globular Clusters:

How dense are clusters? We can measure the density in terms of stars per cubic parsec or stars / parsec 3 Open Clusters: Globular Clusters:

How dense are clusters? We can measure the density in terms of stars per cubic parsec or stars / parsec 3 Open Clusters: ~ stars / pc 3 Globular Clusters: ~ ,000 stars / pc 3 Much higher densities than open clusters!

How dense are clusters? We can measure the density in terms of stars per cubic parsec or stars / parsec 3 Open Clusters: ~ stars / pc 3 Globular Clusters: ~ ,000 stars / pc 3 Much higher densities than open clusters! What did you learn about the density in the cores of star clusters?

How dense are clusters? We can measure the density in terms of stars per cubic parsec or stars / parsec 3 Open Clusters: ~ stars / pc 3 Globular Clusters: ~ ,000 stars / pc 3 Much higher densities than open clusters! What did you learn about the density in the cores of star clusters? In the cores, the density is the greatest

In our neck of the woods... 4 light years Density ~ 1 star/pc 3

4 light years

Density of a globular cluster compared to density in the Solar neighborhood? Density in the Solar neighborhood The density at the centers of star clusters can be up to...

Density of a globular cluster compared to density in the Solar neighborhood? Density in the Solar neighborhood ~ 1 star/pc 3 The density at the centers of star clusters can be up to ~ 100,000 star/pc 3 So, at the center of a globular clusters, the density can be as much as 100,000 times the density around the Sun

The Sun will probably never interact with another star, because … What about in the cores of globular clusters? Density of a globular cluster compared to density in the Solar neighborhood?

The Sun will probably never interact with another star, because there are so few nearby (low density of stars) What about in the cores of globular clusters? In the cores of globular clusters, however, the density is so high that stars have close encounters - stars can interact with each other, even collide in globular clusters Density of a globular cluster compared to density in the Solar neighborhood?

How can we determine the ages of star clusters?

By creating and analyzing the HR diagram for the stars in the cluster We will return to this idea soon!

Why are Globular Clusters so Interesting?

They are extremely dense! At the centers of clusters, stars are sometimes as close together as Pluto is to the Sun! Because of high density, stars interact with each other! - Stars can collide - Complicated binary interactions help create cool binary sources, like X-ray binaries

Why are Globular Clusters so Interesting? In star clusters, all the stars were born at approximately the same time, so they are the same age Like a laboratory for studying stellar evolution They are some of the oldest objects in the Universe Give clues about what things were like then

Why are open clusters young but globular clusters are old?

Star clusters tend to disperse, or evaporate, over time Open clusters are more loosely bound than globulars - Lose stars more easily, in shorter amount of time - Don’t survive more than a few billion years. Globular clusters - more massive, so more tightly bound by gravity, so it takes longer for them to evaporate - Can live for many billions of years

Why are open clusters young but globular clusters are old? Star clusters tend to disperse, or evaporate, over time: Their orbit can take them through the disk of the galaxy - this rips more stars from the cluster

Why are open clusters young but globular clusters are old? So star clusters naturally evaporate over time Star clusters are in orbit around that can take them through the plane of the galaxy - this rips more stars from the cluster Credit: ESO

Why are open clusters young but globular clusters are old? There were many more clusters earlier, but many of them have dissolved out of existence. Credit: ESO