Post on 08-Jul-2020
transcript
1
The Milky Way “Milky Way”: A band of ____ and a _______
Milky Way probably looks likeAndromeda.
The band of light we see isreally 100 billion stars
Milky Way Composite Photo
• ______ in the center• Dark strip in the middle, from _____
Milky Way
Before the 1920’s, astronomers used a “__________model” for the galaxy
Tried to estimate our location in the galaxy by countingstars in different __________
Because some stars are _______ by dust, the trueshape of this group of stars was unclear.
Finding the Center
• Harlow Shapely studied_______ ________.
• He theorized that they mustorbit the true ______ of thegalaxy
A Globular Cluster
Finding the CenterShapely plotted the ________ of the globular star clusters.
He found that they are are not centered on the Sun….
…but are centered on a point about _______ light years fromthe Solar System.
2
The Milky WaySize: The Milky Way isroughly _______ lightyears across, and about_____ light years thick.
Stars: The Milky Way iscomprised of over __________ stars!
Almost everything visiblewith the naked eye isinside the Milky Way
Parts of Our Galaxy
Disk: The ____ Resides in theDisk
Nuclear Bulge: The dense_______ region
Halo: Spherical regionsurrounding the disk where the_______ ________ live.
Parts of OurGalaxy
Milky Way Scales LectureTutorial: Page 123
• Work with a partner or two• Read directions and answer all questions carefully.
Take time to understand it now!• Discuss each question and come to a consensus
answer you all agree on before moving on to the nextquestion.
• If you get stuck, ask another group for help.• If you get really stuck, raise your hand and I will
come around.
Questions:
• How big is the Milky Way?• Where are stars forming (or not forming)?• How much mass is in the Milky Way?• What’s going on at the center?
3
Milky Way: A Spiral Galaxy• Our galaxy seems to be _______: it has spiral arms
• These are dense concentrations of _____ and ____.
• Stars orbit the ___________, pass through thespiral arms as they go.
• Stars ____ _____ andpile up in the spiral arms,like cars in a traffic jam.
The Disk contains ___,so stars are still formingthere. (Population Istars)
The Halo has very little___, and no new starsare forming there.
The halo of the galaxyis populated by ___stars. (Population IIstars)
Star Formation in the Milky Way
Stellar Populations
• Pop. I: Newer, disk & spiral arm stars,with _____ percentage heavy elements
• Pop. II: Older, bulge and halo stars, with_____ percentage of heavy elements
• Heavy elements (metals): anything thatisn’t H, He, or Li
Measuring Distances
•To map the Milky Way Galaxy, we need tomeasure _______ to stars.
• Parallax only works for nearby stars (withinabout ____ light years)
• For more distant stars, we use StandardCandles
Standard Candles
Car Headlights are standard candles: We use them to determine the car’s distance
Standard Candles
• We can easily measure how bright a star appears(________ magnitude)
•If we knew how bright the star really was (its ________magnitude) then we could calculate its distance.
• We need a star whose absolute magnitude is alwaysthe same, wherever we observe it.
• Such a star is called a “standard candle”
4
Cepheid Variables
In 1908, astronomer Henrietta Leavitt discovered a newstandard candle using ______ stars
These stars are called _________
They are named for δ Cephei, thefirst example of such a star.
Henrietta Leavitt
Delta Cephei
Measuring Distances with CepheidsCepheid stars change in brightness.
They pulsate in a very regular way.
Large, bright Cepheids pulsate_____, while small, dim Cepheidspulsate _______.
If we observe the period ofpulsation, we can figure out theabsolute magnitude &luminosity.
If we compare this to the apparentmagnitude, we find the distance!
The Structure of the Milky Way
By measuring the distances to various parts of theMilky Way Galaxy, we map out its structure
The Milky Way is a_______ ______ Galaxy
It has a straight structureat the center called a Bar
Mapping the Milky Way
A modern map of the Milky Way (computer-generated diagram)
The Sun is about_________ out fromthe center
Measuring the Mass of the Milky WayWe use the Sun’s ______around the center of the MilkyWay
The greater the mass insidethe orbit, the ______ the Sunhas move around the center.
This way we can measure themass of the Milky Way.
Total mass: about ___ _______ MSun
Mass of the Milky Way
• The mass of the Milky Way is between ___ billionand MSun and ___ billion MSun
• Stars & Gas we see in the Milky Way can onlyaccount for a fraction of the total mass.
-What is it?
- Why can’t we see it?
5
The Center of the Milky Way The Center of Our Galaxy• The ______ of stars in the Galactic Center is
much greater than in the Sun’s neighborhood.• They appear to be orbiting a ____________ black
hole at the center of the galaxy
Its mass is over__ _____ MSun !
Chapter 13
Galaxies
We now realize that our galaxy is onlyone of billions of galaxies we can see.
These galaxies come in three maintypes:
Spiral, ________ & _______
Spiral Galaxies
M 100 NGC 300
• Typically very bright, _____ in color• Look like _________ (sometimes with
____)
6
… are ____, notflat like spirals
They are typically___ in color.
Less gas and dustthan spirals.
EllipticalGalaxies
Irregular Galaxies• Lack any distinct shape• Are generally ______ than spirals and ellipticals
TheAndromedaGalaxy
Our“Sister”Galaxy
7
Hubble Tuning Fork_______ Galaxies (S): Classified according to spiral arms (a,b,c)
and presence of a bar (“B”)
_______ Galaxies (E): Classified according to shape (E0-E9)
_______ Galaxies(Irr): Basicallyanything funky-looking!
A Barred Spiral Galaxy with only 2 arms.
Measuring Distances toGalaxies
• Too far for ______!
• Standard Candles:– Cepheid Variables (for Local Group)– Type Ia ___________
• Redshifts
Type Ia Supernovae• These are another _______ _____ used to
measure distances to galaxies• Matter from large companion falls onto a white
dwarf, causing its mass to exceed ____ Msun• The resulting explosion is a Type Ia supernova.
Supernovae Types
Type Ia: Exploding White Dwarfin Binary
Type II: “ordinary” supernovaecaused by an explodingmassive stars
Supernovae are Good StandardCandles
• They are allthe same_________
• They can beseen at verylarge________– (1000x
farther thanCepheids)
Supernova in galaxy NGC4526 (HST Image)
Galaxies in Motion• Motion of galaxies is measured using the _______ effect.• Spectrum will be redshifted if it is moving _____,
blueshifted if it is moving _______ us.
Non-moving galaxy spectrum
Redshifted Spectrum
8
Galaxy Redshifts
• When we look at the spectrum of most galaxies,they are __________.
• This means they are moving _____ from us.
• Only a few exceptions: ___________ is movingtowards the Milky Way (may collide with the MilkyWay… in 3 billion years)
Hubble’s Law• E. Hubble measured the ________ to some of these
galaxies (using Cepheid variable stars)• He discovered that the redshift is proportional to the
galaxy’s _________.
•The relationship iscalled Hubble’s Law
Hubble’s Original Data
Hubble’s Law and DistancesHubble’s Law can be used to determine the ______ togalaxies which are too far away for us to see Cepheid
Variables or Type Ia supernovae
vr = H0 x d
d = vr / H0
Hubble Law
d = vr / H0
• d = distance to galaxy (Mpc)• vr = radial velocity of galaxy (km/s)• H0 = Hubble constant (70 km/s/Mpc)
Hubble Law Example: Find the distanceto a galaxy that has a radial velocity of
35,000 km/s.
d = vr / H0
d =
d =
Techniques for Measuring DistancesReview
1. ________
• Measure angle, p. D = 1/p
2. _______ _______ (standard candle)
• Measure Period, get luminosity
3. Type Ia _________ (standard candle)
4. _______ _____
• Measure velocity Vr . Use: Vr = Ho x D
(nearbystars)
(nearestgalaxies)
(distantgalaxies)
(wholeuniverse!)
9
Classifying Galaxies LectureTutorial: Page 127
• Work with a partner or two• Read directions and answer all questions
carefully. Take time to understand it now!• Discuss each question and come to a
consensus answer you all agree on beforemoving on to the next question.
• If you get stuck, ask another group for help.• If you get really stuck, raise your hand and I
will come around.