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Galaxies. Galaxies. A galaxy is a large system of stars and interstellar matter. Stars can be classified by heavy metal concentration. Two types of star populations: Population I – hot, luminous and young stars, seeded with heavy elements
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Galaxies • A galaxy is a large system of stars and interstellar matter. • Stars can be classified by heavy metal concentration. Two types of star populations: Population I – hot, luminous and young stars, seeded with heavy elements Population II – older and cooler with fewer heavy elements (formed earlier than newer stars)
Types of Galaxies • Galaxies can be classified in several ways. The most common is a system developed by Edwin Hubble, who discovered that the universe is filled with many galaxies beyond our own Milky Way.
Types of Galaxies • Galaxies can be classified into four broad categories: 1) spiral 2) lenticular 3) elliptical 4) irregular
Types of Galaxies - Spirals • Spirals are flat disks of stars with bright bulges in their centers. Spiral arms wrap around these bulges. • Spiral arms probably form as the result of waves that sweep through the galactic disk. • In some spirals, a wave organizes the stars in the center into a bar. The arms of barred galaxies spiral outward from the ends of the bar.
Types of Galaxies - Spirals • The nucleus or bulge consists of population II stars, the disk consists of population I stars.
The Milky Way • Our galaxy is a barred spiral. It has at least four spiral arms. • The name was derived from Greek, then Latin.
The Milky Way – Spiral Arms • Four main arms: Perseus, Norma/Cygnus, Crux/Scutum, Carina/Sagittarius • The Sol system is in a spur, the Orion spur.
NGC 1300 Barred Spiral
NGC 3949 Very much like our Milky Way galaxy
NGC 7331 “The Milky Way’s Twin”
M64 A band of absorbing dust make an “evil eye”
M104 – Sombrero Galaxy One of my favourites; I have a summer home there
Sombrero Part 2 Infrared view from Spitzer telescope
Types of Galaxies – Lenticular • Smooth disk galaxies. • Stellar formation in these galaxies has stopped, so they are populated by population II stars.
Types of Galaxies – Elliptical • Like fat, fuzzy eggs or footballs. • Instead of spreading out into a thin disk, as they do in spirals, the stars in ellipticals wrap completely around the galaxy's heart in all directions. • Mostly population II stars
Types of Galaxies – Elliptical • The largest galaxies in the universe are giant ellipticals. • They can contain a trillion stars or more, and span as much as two million light-years (20 times the Milky Way). • Some of them appear to contain "supermassive" black holes at their hearts —three billion times as heavy as the Sun.
NGC 1316 Likely cause by a collision of two gas-rich galaxies
M87 Elliptical galaxy with super massive black hole jet – one that has absorbed over 2 billion solar masses of material
Types of Galaxies – Irregular • Irregulars exhibit peculiar shapes. • Often distorted by gravitation by other objects .
I Zwicky 18 “Baby” galaxy – the youngest galaxy found so far
NGC 1427A Being pulled apart by a nearby galactic cluster
Naming Galaxies • Many conventions exist for naming galaxies. • Conventions include: 1) IAU traditional names 2) Messier object names 3) New General Catalogue names
Hubble Classification Scheme • Elliptical: E elliptical # ellipticity (0 is spherical, 7 is cigar) • Spiral: S Spiral B Barred letter Bulge and Arms
Hubble Classification Scheme • When naming galaxies (especially ellipticals), geometry or viewing angles can play a large role. • The ‘tuning fork’ is not related to galactic evolution.