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Delve into the cosmos as we explore the intricate classification of galaxies based on their shapes and structures, including Spirals, Ellipticals, and Irregulars. Discover the fascinating realm of stellar systems and clusters, as we unravel the mysteries of the Universe through galaxies' appearances and evolution processes.
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Galaxy Classification The Categories • Stellar systems • Stellar Clusters • Galaxies • Galaxy Groups and Clusters • Galaxy Superclusters • The Universe • Galaxies are classified by their appearance • Spiral • Central roundish bulge plus disk • Barred Spiral • Central elongated bulge plus disk • Elliptical • Elongated bulge, no disk • Irregular • No discernible shape
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 or Barred Spirals • Classified by amount of arms, and how tight or loose they are • S0 - no distinguishable spiral arms • Sa, Sb, Sc, Sd - more spiral arms, and looser
S0 Central Bulge Disk • No Spiral Arms
Sa Central Bulge Disk • Tight spiral arms
Sb Central Bulge Disk • Spiral arms
Sc Central Bulge Disk • Loose spiral arms
Sd Central Bulge Disk • Very loose spiral arms
Barred Spirals • Like Spirals, but bulge is oblong • Central bulge, spiral arms • Spiral arms, etc., signs of rotation • Young and old stars, gas, dust • Classified by amount of arms, and how tight or loose they are • SB0 - no distinguishable spiral arms • SBa, SBb, SBc, SBd - more spiral arms, and looser • Milky Way is probably barred spiral SBb
SB0 Central Bar Disk • No spiral arms
SBa Central Bulge Disk • Tight spiral arms
SBb Central Bar Disk • Spiral arms Milky Way?
SBc Central Bar Disk • Looser spiral arms
SBd Central Bar Disk • Very loose spiral arms
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 • Elliptical galaxies have the largest range of masses • Dwarf Spheroidal are the smallest • Giant Ellipticals are the largest Q. 93: Only Old Stars in Elliptical Galaxies
Elliptical Galaxy Shapes • Appearance may depend on angle of view • Amount of flattening probably has to do with rotation
Irregular Galaxies • Gas, dust, young and old stars • Like a galactic disk, no spirals, a mess • Classified as Irr
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
Spiral and Barred Spiral Galaxy Structure • The disk • The bulge • The nucleus • The halo • Globular clusters Q. 94: Looking For Dark Matter in Spiral Galaxies
Dark Matter in Spiral Galaxies • Spirals Rotate • Rotation measured by Doppler shift • Mass, again, not concentrated in the center • 85% of mass is dark matter • Flat rotation curves dark matter
Elliptical Galaxy Structure • The visible part • The nucleus • The halo • Globular clusters
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 85% of the mass of the galaxy is dark matter in the halo
Differences - Spirals vs. Ellipticals • 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?
Hot Gas Vs. Cool Gas • Hot gas has low density • Low density gas cools slowly • Will not cool off in age of universe • Hot gas has pressure: • Gravity vs. pressure = sphere • An elliptical galaxy • Cool gas has high density • High density gas cools more quickly • Can cool off further in short time • Cool gas has little pressure, but still has rotation • Gravity vs. rotation = disk • A spiral (or barred spiral) galaxy
What Determines Galaxy Type? • If we have a source of cool gas: • Gas will form a disk • Disk will form stars • There will be young stars • If all gas is hot: • Gas remains in a giant halo, no disk • No star formation • No young stars • Conclusion: If you have a source of cool gas, you get a spiral or barred spiral, otherwise get elliptical
Galaxy Collisions Why They Matter When two galaxies collide or nearly collide, they can affect each other • The stars themselves do not collide – they are too far apart • Clouds of gas can collide • Bursts of star formation • Gas can get fed to the center (nucleus) of the galaxy • Gas can be heated • The gravity from each galaxy affects the other • May distort the shape of the two galaxies • Galaxies may merge Q. 95: The Most Damaging Galaxy Collisions
Near Misses • As they pass near each other,the gravity of each distorts theother • The slower they pass, the bigger the effect • If unequal in size, smaller galaxy is affected most • There is “tidal friction” which slows down the motion of the two galaxies • Over time, the two galaxies will move closer and closer with each pass • Eventually, a true collision will occur
Galaxy Collisions What happens depends on relative size of the two galaxies Big + Small: • Small galaxy is completely disrupted • Stars slowly absorbed to larger galaxy • This is currently happening to our own galaxy • Sagittarius Dwarf and Canis Major Dwarf - currently getting eaten • Virgo Stellar Stream – Former galaxy being digested Two Equal sized galaxies: • Resultant galaxy will be irregular, initially • Gas will get heated by collision – depending on speed • Galaxies will settle into elliptical or spiral
Colliding Galaxies MW - Andromeda Collision