Stars● Stars are classified by

spectral classes

● O,B,A,F,G,K,M

– Oh be a fine girl (or guy), kiss me!

● Key here is the effective temperature of the star (surface temperature)

– Difference in spectrum is due to temperature on the surface of the star

– See color spectrum of O,B,A,F,G,K, and M in the image to the right ->

Stars

For Jupiter: M/Msun = 0.001

Stars● Special classes (Right Now!)

– R-class stars

● Like K-class except lots of carbon– N-class stars

● Like M-class except lots of carbon– S-class stars

● Like M-class except with zirconium oxide and lanthanum oxide bands

– WN and WC are Wolf-Rayet stars

● Like O-class except carbon and nitrogen emission lines (strong)

Stars● We can determine some

characteristics of stars using simple physical law

– Distance

● Discussed in “lecture 3”● Standard candle

(Cepheid Variable)– Temperature

● Wien's Law● Spectral Class

– Luminosity

● Use inverse square law and apparent brightness

● Class of spectrum

Stars● Stellar spectrum can look

different for the same spectral class of stars

– Some have broader lines then others

– This means different properties in the same spectral class

● To resolve this another class was developed

– Luminosity Class

● Same temperature● Different luminosity

Stars

● The Hertzsprung-Russell Diagram (H-R)

– Grand graph that relates spectral class, luminosity, temperature, mass, size, etc.

– Useful for understanding stars evolution

– Majority of stars are in the main sequence

● Our own Sun

Stars

● Another example of the Hertzsprung-Russell Diagram (H-R)

– Wikipedia's example

– Here you see the different stars as points

– Note the majority (~90%) of points are in the main sequence

Stars

● The spectral temperature luminosity graph is like a Hertzsprung-Russell Diagram (H-R) but is usually for smaller samples like a cluster or galaxy

– Wikipedia's example

– Similar to HR diagram, but is different and requires a conversion from one to another

Stars● Important to the H-R

diagram

– Apparent magnitude

● What we see● Dimmer if further

away– Absolute magnitude

● Total energy output per second

● Apparent magnitude if the object is 10 parsecs away

– Parsec is about 3.262 light-years

Stars

● Periodic gives absolute magnitude

● With absolute magnitude and apparent magnitude can figure out distance of stars

– As objects get more distance their brightness will dim

– Inverse square law

Betelgeuse (example star)● Betelgeuse

– Alpha Orionis (in Orion) (Arabic derived name)

– Red massive supergiant

– Nearing end of life

– Will be a supernova

– If this star was in our solar system it surface would be passed Jupiter

● Image is the first of a surface of a star other than our sun (~2010)

● Interferometry was used to get this image

● Other star’s surfaces have been imaged since

Betelgeuse spectrum● Betelgeuse spectrum

– Upper end of spectrum

https://www.fas.harvard.edu/~astrolab/spectraBetelgeuseRigel_spu21.html

By Raymond Gilchrist - The spectrum appears on a website entitled, "Betelgeuse Graph and Spectrum"

Example Stars● Antares

– Red supergiant

– One of brightest stars in the sky

– In Scorpius (Alpha Scorpii)

– Nebula around the star was expelled by Antares

– Companion blue star

– Lower left corner

● Also shown Rho Ophiuchi (star with blue surrounding it) and Sigma Scorpii (Cephei-type star with red surrounding it)

Example Stars

● Canopus – One of brightest

stars in the sky– In Carina (Alpha

Carinae)– Supergiant

Example Stars● Aldebaran

– In Taurus (Alpha Tauri)

– Means “the follower” from Arabic because if follows the Seven Sisters across the sky

– Giant

– Could have solar system

Example Stars

● Mira– Red Giant Star– Companion is a

white dwarf star possibly causing the strange perturbation in the picture

Example Stars

● Sirius B

– White Dwarf

– Companion to the brighter Sirius-A

– Not known until 1862, however predicted by the motion of Sirius-A

– Diameter 8400 km (smaller than the Earth, however as massive as the Sun)

Example Stars (Main sequence)

● Achenar (in Eridanus)

– Derived from Arabic

– Alpha Eridani

– Blue-white

– Oblate Star...spins very rapidly!

– Bright star, but in the Southern Hemisphere...

Example Stars (Main sequence)

● Altair (Atair)– In Aquila (Alpha

Aquilae)

– 12th brightest star– Oblate because of

rapid rotation– White star

Example Stars (Main Sequence)

● Barnard's Star– In Ophiuchus– Large proper

motion – Discovered by

E.E. Barnard– Red dwarf– Flared in 1998, so

might be a “flare star”

Example Stars (Main Sequence)

● Proxima Centauri– Red Dwarf– Flare Star– Nearest star to

the Sun– Part of Alpha

Centauri system

Evolution of a star (quickly)● Interstellar medium (dust)

● Contraction of the Cloud

● Proto-star

● Young star

● Medium star

● Mature star (contracts – almost all helium now)

● Red Giant

● Helium Flash

● Helium Star

● Bye-Bye

– White Dwarf (small stars)

– Supernova (medium to large stars)

● Remnants may become black dwarfs● Or black holes