Presentation on theme: "Galaxy Classification"— Presentation transcript:
1 Galaxy Classification Classifying galaxies is hard, and different people do it different waysExcept for nearby, we can only see bright galaxiesI will ignore dwarf spheroidal (dSph) and dwarf elliptical (dE) classificationsGalaxies come in four broad categories, based on their appearanceSpiralCentral roundish bulge plus diskBarred SpiralCentral elongated bulge plus diskEllipticalElongated bulge, no diskIrregularNo discernible shape
2 Spiral galaxies Pinwheel-like Central Bulge, spiral arms Spiral arms, etc., signs of rotationYoung and old stars, gas, dust80% of large galaxies are SpiralsClassified by amount of armsS0 - no spiral armsSa, Sb, Sc, Sd - more spiral arms
8 Barred Spirals Like Spirals, but bulge is oblong Central Bulge, spiral armsSpiral arms, etc., signs of rotationYoung and old stars, gas, dustMilky Way is probably barred spiral SBbClassified by amount of armsSB0 - no spiral armsSBa, SBb, SBc, SBd - more spiral arms
29 Elliptical HalosElliptical galaxies don’t have thick clouds, but they do have diffuse, hot gasThese gasses emit X-raysGravity vs. pressure – they expand to make a giant sphereAmount 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. EllipticalsSpirals appear to have more spinSpirals have disks and spiral structureSpirals have dust/gas/young stars in the diskEllipticals have hot gas spread out through a large haloCan 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 centerIn an elliptical galaxy, any loose gas and dust falls into the dense centerStar formation gets very fastLots of supernovasLoose gas gets swept out to haloNo more star formation
32 Near Miss Collisions Galaxy Collisions When two galaxies collide or nearly collide, they can affect each otherMostly through gravitational interactionsNear Miss CollisionsAs they pass near each other, the gravity of each distorts the otherThe slower they pass, the bigger the effectIf unequal in size, smaller galaxy is affected mostTidal heating – energy is transferred from net motion to internal motion of starsStar motions get more randomizedEnergy comes from kinetic energy of orbit – orbit loses energy and galaxies move closer togetherOver time, the two galaxies will move closer and closer with each passEventually, a true collision will occur
33 Actual Galaxy Collisions What happens depends on relative size of the two galaxiesBig + Small:Small galaxy is completely disruptedStars enter large galaxyOver time, they get absorbedThis is currently happening to our own galaxySagittarius Dwarf and Canis Major Dwarf – currently being disruptedVirgo Stellar Stream – a dead galaxy whose stars are being absorbedTwo Equal sized galaxies:Resultant galaxy will be irregular, initiallyBased on total spin, resultant galaxy eventually settles down to spiral or elliptical
39 Giant Elliptical Galaxies Sometimes many galaxies collide and mergeIf large numbers collide, spin cancels outGiant galaxy becomes an ellipticalGiant Elliptical
40 Looking Out = Looking Back Light travels at about 0.3 pc per yearThe farther away you are looking, the longer ago you are seeing1 kpc 3.3 ky1 Mpc 3.3 My1 Gpc 3.3 GyrYou can see back almost to the beginning of the Universe!Galaxies in the past: how do they differ?Galaxies long agoSmaller than modern galaxiesIrregulars are more commonWhy?Galaxies collided a lot in the pastMany irregulars from recent collisionsGalaxies got bigger from mergers
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