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Galaxy Classification

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Presentation on theme: "Galaxy Classification"— Presentation transcript:

1 Galaxy Classification
Classifying galaxies is hard, and different people do it different ways Except for nearby, we can only see bright galaxies I will ignore dwarf spheroidal (dSph) and dwarf elliptical (dE) classifications Galaxies come in four broad categories, based on their appearance Spiral Central roundish bulge plus disk Barred Spiral Central elongated bulge plus disk Elliptical Elongated bulge, no disk Irregular No discernible shape

2 Spiral galaxies Pinwheel-like Central Bulge, spiral arms
Spiral arms, etc., signs of rotation Young and old stars, gas, dust 80% of large galaxies are Spirals Classified by amount of arms S0 - no spiral arms Sa, Sb, Sc, Sd - more spiral arms

3 S0 Central Bulge Disk No Spiral Arms

4 Sa Central Bulge Disk Tight spiral arms

5 Sb Central Bulge Disk Spiral arms

6 Sc Central Bulge Disk Loose spiral arms

7 Sd Central Bulge Disk Very loose spiral arms

8 Barred Spirals Like Spirals, but bulge is oblong
Central Bulge, spiral arms Spiral arms, etc., signs of rotation Young and old stars, gas, dust Milky Way is probably barred spiral SBb Classified by amount of arms SB0 - no spiral arms SBa, SBb, SBc, SBd - more spiral arms

9 SB0 Central Bar Disk No spiral arms

10 SBa Central Bar Disk Tight spiral arms

11 SBb Central Bar Disk Spiral arms Milky Way?

12 SBc Disk Central Bar Looser spiral arms

13 SBd Central Bar Disk Very loose spiral arms

14 Elliptical Galaxies Look like a sphere or a flattened sphere
Little gas and dust Mostly old stars Classified by how round they look E0 looks circular E7 is very elongated

15 E0

16 E1

17 E2

18 E3

19 E4

20 E5

21 E6

22 E7

23 Elliptical Galaxy Shapes
Appearance may depend on angle of view Amount of flattening probably has to do with rotation

24 Irregular Galaxies Gas, dust, young and old stars
Like a galactic disk, no spirals, a mess Classified as Irr

25 Hubble’s Tuning Fork Spirals, Barred Spirals, and Ellipticals fit together in a pattern called the Tuning fork Diagram S0 - Sa - Sb - Sc - Sd E0-E1-E2-E3-E4-E5-E6-E7 SB0-SBa-SBb-SBc-SBd

26 What determines a galaxy’s classification?
It’s not size or mass Rotation! Rotation measured by Doppler shift Most mass is dark matter, again To Observer

27 Spiral Galaxy Structure
Disk Bulge Nucleus Halo Globular Clusters

28 Elliptical Galaxy Structure
Visible Part Nucleus Halo

29 Elliptical Halos Elliptical galaxies don’t have thick clouds, but they do have diffuse, hot gas These gasses emit X-rays Gravity vs. pressure – they expand to make a giant sphere Amount of gravity tells us 90% of the mass of the galaxy is dark matter in the halo

30 Can we explain these differences?
Differences - Spirals vs. Ellipticals Spirals appear to have more spin Spirals have disks and spiral structure Spirals have dust/gas/young stars in the disk Ellipticals have hot gas spread out through a large halo Can we explain these differences?

31 The Effects of Spin Spin causes flattening – formation of a disk
And disks naturally form spiral structure! Rotation prevents the gas and dust from all falling to the center In an elliptical galaxy, any loose gas and dust falls into the dense center Star formation gets very fast Lots of supernovas Loose gas gets swept out to halo No more star formation

32 Near Miss Collisions Galaxy Collisions
When two galaxies collide or nearly collide, they can affect each other Mostly through gravitational interactions Near Miss Collisions As they pass near each other, the gravity of each distorts the other The slower they pass, the bigger the effect If unequal in size, smaller galaxy is affected most Tidal heating – energy is transferred from net motion to internal motion of stars Star motions get more randomized Energy comes from kinetic energy of orbit – orbit loses energy and galaxies move closer together Over time, the two galaxies will move closer and closer with each pass Eventually, a true collision will occur

33 Actual Galaxy Collisions
What happens depends on relative size of the two galaxies Big + Small: Small galaxy is completely disrupted Stars enter large galaxy Over time, they get absorbed This is currently happening to our own galaxy Sagittarius Dwarf and Canis Major Dwarf – currently being disrupted Virgo Stellar Stream – a dead galaxy whose stars are being absorbed Two Equal sized galaxies: Resultant galaxy will be irregular, initially Based on total spin, resultant galaxy eventually settles down to spiral or elliptical

34 Colliding Galaxies





39 Giant Elliptical Galaxies
Sometimes many galaxies collide and merge If large numbers collide, spin cancels out Giant galaxy becomes an elliptical Giant Elliptical

40 Looking Out = Looking Back
Light travels at about 0.3 pc per year The farther away you are looking, the longer ago you are seeing 1 kpc  3.3 ky 1 Mpc  3.3 My 1 Gpc  3.3 Gyr You can see back almost to the beginning of the Universe! Galaxies in the past: how do they differ? Galaxies long ago Smaller than modern galaxies Irregulars are more common Why? Galaxies collided a lot in the past Many irregulars from recent collisions Galaxies got bigger from mergers

41 Galaxies Long Ago

42 Long, Long Ago


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