August 8 , AST201H1
Why are we unlikely to find Earth- like planets around halo stars in the galaxy?
- B. Halo stars formed from gas containing few heavy elements, the elements
needed to create terrestrial planets like the earth.
Edge on view. Bulge is center. Halo outside. (learn diagram)
In the disk, stars of all ages, many gas clouds.
Spheroidal component: bulge and halo, old stars, few gas clodus.
- In the direction of the constellation Sagittarius
Red pattern shows characteristics, connect the brightest stars.
What lies in the center of our galaxy? Orbits of stars indicate a mass of about 4
million Msun. Violent. Lots of gas.
Galaxies and cosmology.
Hubble deep field. Long exposure. To allow photons to come in. Thought it was just
black, but a lot of galaxies. Comes in different shapes and colors, just like stars.
3 main types
- Spiral (milky way)
Mainly dependent on shape. Shape determines physical processes.
M74- a grand design spiral.
Pockets of blue light star formation.
Hoag’s object- a ring galaxy. Used to be a spiral. Hit head on with a small and
compact galaxy. Punched through the center, caused shock that destroyed spirals.
Opposite directions. M64- gas spinning in two different directions.
M87- giant elliptical galaxy. Large blob. No disk.
M82- a starburst galaxy (irregular galaxy type)
Give any type of shape. Sometimes called cigar. Infrared are picking up cooler
component made of dust and gas
Irregular are smaller.
Galaxies- a lot of stars banded together by gravity.
Hubble’s turning fork classification scheme. (not evolution) Elongated shape.
The cosmic distance ladder (how do we measure the distances to galaxies?)
Parallax. Within 200 pc. Only nearby stars.
Variable Stars (Cepheids), Supernova, Hubble’s law
Distance + apparent brightness luminosity Or
Luminosity + apparent brightness distance
A standard candle is an object whose luminosity we can determine without
measuring its distance. That particular class of object outputs the same energy no
matter how far away. We know that they will always emit the same amount of
energy per unit time.
Rung 4: Cepheid variable stars
Have very regular periodic changes of their brightness.
Brightness peaks (measure the period) Pulsation.
Pressure and gravity cannot balance: star expands and contracts regularly
(changing size changing brightness)
Light curve. (low)
If we find a Cepheid with a measured period of 30 days… the period-luminosity
relation tells us that its luminosity is about 10,000 Lsun.
Luminosity (Lsun) Priod (days)
How luminosity changes when period changes.
Take the apparent brightness, take the period, to get luminosity. When we have the
period, we put the period down on the x axis, the period is 10 days, go to that and
then read the luminosity. With luminosity we get the distance.
Rung 6: Supernova
Supernova Type 1a
White dwarf binary system, mass from the companion will start giving material.
When a certain point is reached, huge detonation. 1.4.
So bright that we can observe in very distant galaxies.
Excellent “standard candles”.
Luminosity and how it changes with time. Red curve.
Rung 7: Hubble’s law
Velocity vs distance. Each point is a galaxy. Telling us how velocity is changing with
distance. Close stars, velocity is low. Further away, velocity is higher.
B is moving away because red shifted
Solved the puzzle of “Spiral Nebulae”
Can’t separate the objects of the stars. Catalogues have everything. Any smudge, he
put it into the catalogue. Until Hubble came into the scene, we didn’t know where
this spiral nebulae are. Are they galaxies or not?
Hubble found that the spectral features of most of the galaxies are redshifted.
Spectrum are absorption, some emission. Both. Shifted towards longer wavelengths.
Apparent brightness versus wavelength.
Established a relationship between redshift and distance from the milky way.
Z is proportional to distance.
Hubble thought that he was measuring Doppler shirts, indicating that distant
galaxies were moving at high speeds. Translated this redshift into velocities, velocity of recession. Type of motion that
indicates that things move away from us.
Speed of recession versus distance from MW. The further away the galaxies are the
faster the galaxies are moving away. But this is wrong!
The Local Group- a few dozen galaxies orbiting their common center of mass.
Collide. Soon. Hubble’s Law tells