Lesson 10: The Milky Way, The History of the Milky Way, the Galactic Centre Flashcards
How does the Milky Way appear in our night sky? Describe how it appears.
- The Milky Way Galaxy appears in our sky as a faint band of light
- Dusty gas clouds obscure our view because they absorb visible light
○ The dusty gas is where stars form
What is the interstellar medium?
Interstellar medium: region between the stars that contains vast, diffuse clouds of gases and minute solid particles
- makes new star systems
How did William and Caroline Herschel create the first map of the Milky Way? What did this map lead people to believe? What assumption did they make that was
incorrect?
William and Caroline Herschel (brother/sister), made the first attempt to map the Milky Way, by estimating distances to stars by their brightness.
- Based on the above map, astronomers more or less believed we were at the centre of the Milky Way
Who was Henrietta Swan Leavitt? Describe what she discovered about variable stars?
Henrietta Swan Leavitt was part of the Harvard College Observatory team of ‘computers’ working on stellar spectra.
Leavitt discovered over 1700 variable stars like delta Cephei (changes brightness periodically), 20 of which were in the Small Magellanic Cloud (dwarf galaxies that orbit our galaxy; collection of stars)
Remember, we cannot measure actual luminosity off the sky, only apparent magnitude.
How was Swan Leavitt able uncover the Period-Luminosity relationship of Cepheid variables without knowing the actual Luminosities of the stars?
Or without knowing the actual distance to any of them?
If the 20 variable stars are part of the Small Magellanic Cloud , then we can assume they are all the same distance away! (roughly)
Their differences in apparent brightness reflect their differences in Luminosity
In 1912, Leavitt published an analysis of the 20 Cepheid Variable Stars in the SMC, and found this:
The (apparently) brighter the cepheid was, the longer its period of variability
How are luminosity, apparent brightness, and distance related?
- A Cepheid Variable star that has a LONG period has a larger luminosity.
- A Cepheid Variable star that has a SHORT period, has a smaller luminosity
If you know the Luminosity and you know the apparent brightness, you can easily calculate the distance to the star **Distance = √Luminosity/Brightness **
How did Shapley calibrate Swan Leavitt’s discovery?
In 1917, Harlow Shapley measured the TRUE distances to 93 globular clusters, and found that the Sun is not at the centre of the Milky Way
Using his sample of nearby Cepheids with known distances, he converted Leavitt’s apparent magnitudes **to luminosity (L) **
How do we know that our galaxy has spiral arms? What radio astronomy technique was used to measure the locations of spiral arms in our galaxy?
Hydorgen Atom is stable for about 10 million years
After that:
In a Hydrogen atom, when the *magnetic poles *of the proton and electron (e-) are aligned, the electron eventually flips over.
Protons and electrons can exits with different spins
* Sometime they can spin in the same or opposite direction
If you let a hydrogen atom, it will naturally flip the electron to a less energetic state.. Releases low energy ray of light (radio light).. Radiates at 21 cm
* We see the 21cm emission from large clouds of Hydrogen in our galaxy and are bound to **the arms in our galaxy **
Describe the motions of stars in the Milky Way. Are stars confined to the plane? What are they bounded by?
Stars in the disk all orbit in the same direction, with a little bit of up-and- down motion
Stars can be not be completely confined to the plane of the milky way, but are bound by gravity to move “up and down” around the arms
How can we use the motion of the Sun through the Milky Way to calculate the mass of the Milky Way?
We can use orbiting things to tell the mass of a galaxy
The orbital speed (v) and radius (r) of an object on a circular orbit around the galaxy tells us the mass within that orbit
Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit around the Milky Way
* Takes the sun 230-million years to orbit around milky way
What is dark matter? How do we know it exists and how does Kepler’s laws of planetary motion factor in to its discovery.
Dark matter - just mass we can’t see that are part of the other galaxy (NOT protons and electrons, doesn’t shine)
Kepler laws found that:
Things are rotating around the outer galaxy FASTER than they should be, given the amount of matter within their orbits
Conclusion: either Kepler/ Newtonian physics is wrong, or there is more matter in the Milky Way that we can’t account for
- The Milky Way has about 20x more mass than we can account for (Dark Matter)
Describe the differences between stars in the disk and stars in the halo.
Also, what are the associated elements to the disk population and halo population?
Disk population— stars with compositions like the Sun, orbiting near the planet of the galaxy. A mix of young and old stars
- Primary young stars and heavier elements
Halo population — stars with a scarcity of heavier elements, orbiting at random inclinations to the galactic disk. All older, low-mass stars.
- lighter elements
How do these differences point to how the Galaxy may have formed?
define protogalactic cloud, how does this relate to Halo Stars?
The reason for the difference: the Milky Way’s gas has sunk into the disk!
Protogalactic cloud: a cloud with mass equal to that of a galaxy, and whose collapse leads to the formation of the currently observed stars
Since the oldest stars are in the halo, then it makes sense that the protogalactic cloud that the Milky Way formed was spherical
Halo stars formed in clumps that later merged
- It shouldn’t been a single pocket of star formation rather, pockets of star formation that collide together
- Galaxy was formed by actively eating other galaxies
What is the Monolithic Collapse theory?
“Monolithic Collapse” Theory: states the galaxy formed from one giant protogalactic cloud
What is the Sagittarius Dwarf Galaxy? How does it point to a different theory of galaxy forming?
The Sagittarius Dwarf Galaxy - Active galactic Cannibalism
- A dwarf galaxy is seen plunging through the disk and is leaving a trail behind
- It’ll pierce through the disk and loops around again through the disk again and as its doing this, its slowly loosing stars behind itself
