Lecture 13: Stars and Star Clusters

Question 1

Major Assumptions in Stellar Astrophysics

Answer 1

• If two stars are on the main sequence and have the same spectra they are basically identical
• same mass means same luminosity
• same luminosity means same surface temp which means same radius
• stars move slightly on the main sequence as they age and depends on composition of star

Question 2

Star Clusters

Answer 2

• Group of stars held together by their mutual gravity
• important for understanding of main sequence and models for stellar evolution
• all stars in a cluster are assumed to be roughly the same age
• formed together at the same time from the same gas cloud

Question 3

Types of Star Clusters

Answer 3

Associations
- 100 Stars, probably short-lived because stars escape
Open Clusters
- many thousands of stars
Globular Clusters
- Spherical balls of 100 thousand to a million stars
- Many white dwarfs

Question 4

HR Diagram for Star Clusters

Answer 4

• For a given star cluster, the lifetime of the stars at the top left end of the MS is the age of the cluster
• This is called the turnoff age because it is where stars turnoff the main sequence

Question 5

Using Star Clusters to estimate distances

Answer 5

• Main Sequence fitting
• Offset in brightness between two clusters must be due to distance
• Pleiades is 7.5 times fainter than Hyades, meaning it is root 7.5 times further away

Question 6

Variable Stars

Answer 6

• Many stars vary in brightness for various reasons
• Here we look at stars whose luminosity varies on short timescales
• eclipsing binary stars vary, but not intrinsically
• most common cause of a intrinsic variable star is a pulsation
• Star increases then decreases in radius
• Surface temp also varies
• causes a variety of light curves (luminosity as function of time)

Question 7

Types of Variable Stars: Pulsating

Answer 7

Cepheid
- periods of 3-50 days, vary my 0.2 magnitudes and are very very bright
RR Lyrae
- periods of less than a day, less bright than Cepheids
Mira
- Periods of 80 to 600 days, large range in brightness

Question 8

Other types of Variable Stars

Answer 8

Eruptive Variables
- flare stars
- Novae
- Supernovae
- Very bright but very rare
Rotating Spotted Stars

Question 9

Variable Star light curves

Answer 9

• light curve is plot of brightness of light at different times
• many different shapes of light curves are found
• shape tells us a lot about the kind of variations in radius and surface temperature that the star is undergoing

Question 10

Period-Luminosity Law of Cepheids

Answer 10

• Cepheid variable stars have a well-behaved and simple relationship between their period and their luminosity
• Provides us with a standard candle, an object with well determined luminosity
• Can find distance if luminosity and brightness are known

Question 11

Novae

Answer 11

• Very rarely, a star will become much brighter
• the original star usually isn’t known before the event so novae means new star
• some stars will repeat this behaviour decades or centuries later

Question 12

Supernovae

Answer 12

• Seen and recorded though history
• Last naked eye supernova in 1604
• modern study based on those seen in other galaxies
• estimated that each large galaxy has one supernova every one to three hundred years
• ## brighter than all of the rest of the stars in its galaxy combined together

Question 13

Supernova Light Curves

Answer 13

• Type 1 and Type 1a have identical light curves and identical peak luminosity
• Great standard candles because they are very very bright and identical to each other

Question 14

Supernova Remnants

Answer 14

• Results in Nebulas