Star Cluster Trivia!Star Cluster Trivia!

Open ClustersOpen Clusters

HyadesHyades PleiadesPleiades

Globular ClustersGlobular Clusters

Omega CentauriOmega Centauri M15M15

What are star clusters?What are star clusters?

What are star clusters?What are star clusters?

GroupGroup of stars, bound together of stars, bound together by mutual gravitational pull by mutual gravitational pull among all starsamong all stars

Describe main differences Describe main differences between Open and Globular between Open and Globular ClustersClusters

SizeSize

ShapeShape

Describe main differences Describe main differences between Open and Globular between Open and Globular ClustersClusters

SizeSizeOpen clusters have fewer stars than Open clusters have fewer stars than globular clustersglobular clustersOpen clusters are more sparse, globulars Open clusters are more sparse, globulars have higher density or concentration of have higher density or concentration of starsstars

ShapeShapeglobular clusters are very spherical and globular clusters are very spherical and symmetricsymmetricopen clusters are more irregularopen clusters are more irregular

How many stars do they How many stars do they have?have?

Open ClustersOpen Clusters

Globular ClustersGlobular Clusters

Open ClustersOpen Clustershundreds to thousandshundreds to thousands~100 - 1000~100 - 1000

Globular ClustersGlobular Clustershundreds of thousands to millionshundreds of thousands to millions~100,000 to 1,000,000~100,000 to 1,000,000

How many stars do they How many stars do they have?have?

How many clusters are How many clusters are there in the Milky Way?there in the Milky Way?

Open:Open:

Globular:Globular:

How many clusters are How many clusters are there in the Milky Way?there in the Milky Way?

Open:Open:over 1100 known, may be more (up to over 1100 known, may be more (up to 10,000)10,000)

Globular:Globular:~~150, may be more150, may be more

There may be There may be many more clustersmany more clusters than we than we know of so far - they can be know of so far - they can be hidden by clouds hidden by clouds of gas and dustof gas and dust in the Galaxy! in the Galaxy!

How old are the clusters in How old are the clusters in the Milky Way?the Milky Way?

Open:Open:

Globular:Globular:

How old are the clusters in How old are the clusters in the Milky Way?the Milky Way?

Open:Open:less than a billion years old less than a billion years old (typically a few (typically a few hundred million years old)hundred million years old)

Globular:Globular:10-13 billion years old 10-13 billion years old ((much oldermuch older than than open clusters)open clusters)

Open Clusters:Open Clusters:

Globular Clusters:Globular Clusters:

http://www.atlasoftheuniverse.com/globular.html

50 50 kiloparseckiloparsec

ss

Where do star clusters reside Where do star clusters reside within the Milky Way?within the Milky Way?

Where do star clusters reside Where do star clusters reside within the Milky Way?within the Milky Way?

Open Clusters:Open Clusters:Disk or plane of Disk or plane of the Galaxythe Galaxy

Globular Clusters:Globular Clusters:Halo of the GalaxyHalo of the Galaxy

http://www.atlasoftheuniverse.com/globular.html

HaloHaloGlobular Globular ClustersClusters

DiskDisk

BulgeBulgeGalactiGalacti

c c NucleusNucleus

SunSun

50 50 kiloparseckiloparsec

ss

How dense are clusters?How dense are clusters?

We can measure the density in terms ofWe can measure the density in terms of

Open Clusters:Open Clusters:

Globular Clusters:Globular Clusters:

How dense are clusters?How dense are clusters?

We can measure the density in terms ofWe can measure the density in terms ofstars per cubic parsecstars per cubic parsec or or stars / parsecstars / parsec33

Open Clusters:Open Clusters:

Globular Clusters:Globular Clusters:

How dense are clusters?How dense are clusters?

We can measure the density in terms ofWe can measure the density in terms ofstars per cubic parsecstars per cubic parsec or or stars / parsecstars / parsec33

Open Clusters:Open Clusters:~ 0.1 - 50 stars / pc~ 0.1 - 50 stars / pc33

Globular Clusters:Globular Clusters:~ 1 - 100,000 stars / pc~ 1 - 100,000 stars / pc33

Much higher densities than open clusters!Much higher densities than open clusters!

How dense are clusters?How dense are clusters?

We can measure the density in terms ofWe can measure the density in terms ofstars per cubic parsecstars per cubic parsec or or stars / parsecstars / parsec33

Open Clusters:Open Clusters:~ 0.1 - 50 stars / pc~ 0.1 - 50 stars / pc33

Globular Clusters:Globular Clusters:~ 1 - 100,000 stars / pc~ 1 - 100,000 stars / pc33

Much higher densities than open clusters!Much higher densities than open clusters!

What did you learn about the density in the What did you learn about the density in the cores of star clusters?cores of star clusters?

How dense are clusters?How dense are clusters?

We can measure the density in terms ofWe can measure the density in terms ofstars per cubic parsecstars per cubic parsec or or stars / parsecstars / parsec33

Open Clusters:Open Clusters:~ 0.1 - 50 stars / pc~ 0.1 - 50 stars / pc33

Globular Clusters:Globular Clusters:~ 1 - 100,000 stars / pc~ 1 - 100,000 stars / pc33

Much higher densities than open clusters!Much higher densities than open clusters!

What did you learn about the density in the What did you learn about the density in the cores of star clusters?cores of star clusters?In the cores, the density is the greatestIn the cores, the density is the greatest

In our neck of the In our neck of the woods...woods...

4 light years4 light years

Density Density ~ 1 star/pc~ 1 star/pc33

4 light years4 light years

4 light years4 light years

4 light years4 light years

Density of a globular cluster Density of a globular cluster compared to density in the Solar compared to density in the Solar neighborhood?neighborhood?

Density in the Solar neighborhoodDensity in the Solar neighborhood

The density at the centers of star clusters can be up to... The density at the centers of star clusters can be up to...

Density of a globular cluster Density of a globular cluster compared to density in the Solar compared to density in the Solar neighborhood?neighborhood?

Density in the Solar neighborhoodDensity in the Solar neighborhood~ ~ 1 star/pc1 star/pc33

The density at the centers of star clusters can be up to The density at the centers of star clusters can be up to ~ ~ 100,000 star/pc100,000 star/pc33

So, at the center of a globular clusters, the density can So, at the center of a globular clusters, the density can be as much as be as much as 100,000 times the density around 100,000 times the density around the Sunthe Sun

The Sun will probably never interact with The Sun will probably never interact with another star, because …another star, because …

What about in the cores of globular clusters?What about in the cores of globular clusters?

Density of a globular cluster Density of a globular cluster compared to density in the Solar compared to density in the Solar neighborhood?neighborhood?

The Sun will probably never interact with The Sun will probably never interact with another star, because another star, because there are so few there are so few nearby (low density of stars)nearby (low density of stars)

What about in the cores of globular clusters?What about in the cores of globular clusters? In In the cores of globular clusters, however, the the cores of globular clusters, however, the density is so high that density is so high that stars have close stars have close encountersencounters - stars can interact with each other, even - stars can interact with each other, even collide in globular clusterscollide in globular clusters

Density of a globular cluster Density of a globular cluster compared to density in the Solar compared to density in the Solar neighborhood?neighborhood?

How can we determine How can we determine the ages of star the ages of star clusters?clusters?

How can we determine How can we determine the ages of star the ages of star clusters?clusters?

By creating and analyzing the By creating and analyzing the HR diagram HR diagram for the stars in the clusterfor the stars in the cluster

We will return to this idea soon!We will return to this idea soon!

Why are Globular Why are Globular Clusters so Interesting? Clusters so Interesting?

Why are Globular Why are Globular Clusters so Interesting? Clusters so Interesting?

They are extremely dense!They are extremely dense!At the centers of clusters, stars are At the centers of clusters, stars are sometimes as close together as Pluto is sometimes as close together as Pluto is to the Sun!to the Sun!

Because of high density, stars interact Because of high density, stars interact with each other!with each other!- Stars can collide- Stars can collide- Complicated binary interactions help - Complicated binary interactions help create create cool binary sources, like X-ray binariescool binary sources, like X-ray binaries

Why are Globular Why are Globular Clusters so Interesting? Clusters so Interesting?

In star clusters, all the stars were born at In star clusters, all the stars were born at approximately the same time, so they are approximately the same time, so they are the same agethe same ageLike a laboratory for studying stellar Like a laboratory for studying stellar evolutionevolution

They are some of the oldest objects in the They are some of the oldest objects in the UniverseUniverseGive clues about what things were like Give clues about what things were like thenthen

Why are open clusters Why are open clusters youngyoung but globular but globular clusters are clusters are oldold??

Why are open clusters Why are open clusters youngyoung but globular but globular clusters are clusters are oldold??Star clusters tend to Star clusters tend to dispersedisperse, or , or evaporateevaporate, over , over timetime

Open clustersOpen clusters are more are more loosely boundloosely bound than than globularsglobulars- Lose stars more easily, in shorter amount of time- Lose stars more easily, in shorter amount of time- Don- Don’’t survive more than a few billion years.t survive more than a few billion years.

Globular clustersGlobular clusters - - more massivemore massive, so , so more more tightly boundtightly bound by gravity, so it takes longer for by gravity, so it takes longer for them to evaporatethem to evaporate- Can live for many billions of years- Can live for many billions of years

Why are open clusters Why are open clusters youngyoung but globular but globular clusters are clusters are oldold??Star clusters tend to Star clusters tend to dispersedisperse, or , or evaporateevaporate, over time:, over time:Their orbit can take them through the Their orbit can take them through the diskdisk of the galaxy of the galaxy - this rips more stars from the cluster- this rips more stars from the cluster

Why are open clusters Why are open clusters youngyoung but globular but globular clusters are clusters are oldold??

So star clusters naturally evaporate over timeSo star clusters naturally evaporate over time

Star clusters are in orbit around that can take Star clusters are in orbit around that can take them through the plane of the galaxy - this rips them through the plane of the galaxy - this rips more stars from the clustermore stars from the cluster

Credit: ESOCredit: ESO

Why are open clusters Why are open clusters youngyoung but globular but globular clusters are clusters are oldold??

There were many more There were many more clusters earlier, but many clusters earlier, but many of them have dissolved of them have dissolved out of existence.out of existence.

Credit: ESOCredit: ESO