Star Clusters A cluster is a group of stars in a small volume of space, held together by the gravity of the stars. Because of their closeness to one another, we know that these stars all formed at the same time, from the same nebula. all the same ageTherefore, the stars are all the same age.
Star Clusters The stars may all be the same age, but since they have different masses, they go through their lives at different rates. Therefore, in a cluster, we see stars at all different stages of their development.
2 Types of Clusters Open Clusters (examples: Pleiades, Wild Duck Cluster) – a few hundred stars in a sphere about 10 LY in diameter. This means that there is only about 1 star / cubic light year. Low Density All types of stars may be observed, from large blue supergiants to red dwarfs.
More about open clusters Open clusters are also called “galactic” clusters, because they occur in the plane of the galaxy. The stars in open clusters are called “Population I” stars.
2 Types of Clusters Globular Clusters (examples: M13 in Hercules, Omega Centauri) – a million stars in a sphere about 100 LY in diameter. Density of stars is much higher: 2-4 stars per cubic light year.
Globular Clusters Not all types of stars are observed in these clusters, only yellow, orange, & red stars. The brightest stars are red giants. What does this tell you about the age of globular clusters? They must be old enough for the larger, hotter stars to have died.
More about globular clusters Globular clusters occur all around & within the galaxy. They were formed when the galaxy formed, and orbit around the galaxy’s center of mass. The stars in globular clusters are called “Population II” stars because they were studied after open clusters. Population II stars are very poor in heavy elements. What’s this mean?
HR Diagram of Clusters The HR Diagram is like a “family portrait” of the stars. It shows stars of all ages, in all their stages of development, from birth through old age & death. What would HR diagrams of open & globular clusters look like?
Luminosity Temperature A young cluster – all stars are on the main sequence.
Temperature Luminosity In an older cluster, the largest stars have evolved off the main sequence, into red supergiants. this point is the “turnoff point”
Temperature Luminosity The older the cluster, the farther down the main sequence line the turnoff point is found.
Temperature Luminosity In really old clusters, the HR diagram takes on a characteristic “sickle” shape. The hook at the top is stars that have begun to shed their outer layers as planetary nebulas.
M5 – a very old globular cluster
More Evidence – Variable stars Many stars in our sky are variable, meaning that their brightness changes over time, sometimes in a regular way, sometimes irregularly. These stars are near the end of their red giant phases, when the thermal pulses have started.
3 important types of variables Cepheid Variable Stars RR Lyrae Stars Irregular red giants
Cepheid Variable Stars Named after the first star of this type discovered: -Cepheii (Cepheus constellation). These stars pulsate in a very regular way, with pulsation periods of 5 to 20 days. The shorter periods (5-10 days) are called Type I cepheids. The longer periods (11-20 days) are called Type II cepheids.
Cepheid Variable Animation The star swells, then shrinks, growing brighter, then dimmer! http://imgsrc.hubblesite.org/hu/db/1994/49/ videos/b/formats/low_mpeg.mpghttp://imgsrc.hubblesite.org/hu/db/1994/49/ videos/b/formats/low_mpeg.mpg
Why are cepheids so important? Besides giving evidence of stellar evolution, they are also good distance markers, even to other galaxies! The period of a cepheid variable is directly linked to its average brightness: the longer the period, the brighter the star. This gives us m & M, and that let’s us calculate distance: 10 ((m-M+5) 5) We’ll do a lab on this a little later.
A cepheid in another galaxy http://hubblesite.org/newscenter/newsdesk /archive/releases/2003/24/video/bhttp://hubblesite.org/newscenter/newsdesk /archive/releases/2003/24/video/b
RR Lyrae Stars These are old Population II stars, often found in globular clusters. All RR Lyrae stars have about the same pulsation period (12 hours). All RR Lyrae stars have about the same brightness: 50 x brighter than our sun, or M = +3.2 These are great distance markers too!
Irregular variables Many red giants & supergiants don’t pulsate with a regular pattern – they’re too unstable internally. Pulsations can be as long as hundreds of days. These stars can’t be used as distance markers because there’s no relationship between their brightness & their rate of pulsation.
Do we have enough evidence? We can observe every different stage of a star’s life: nebula, protostar, pre-main-sequence star, main sequence star, red giant, thermal pulses, planetary nebula, and finally white dwarf. Next, we’ll look at how really big stars live & die.