chapter 25.1 architecture of galaxy & 25.2 spiral structure & the mass of Galaxy & the centre of the Galaxy & stellar populations in Galaxy & 25.6 formation of Galaxy

features of Galaxy

• brightest part of Galaxy: consists of a thin, circular, rotating disk of stars distributed across a region about 100,000 light-years in diameter and about 2000 light years thick

• youngest stars and the dust and gas from which stars form, are found within 100 light years of the plane of the Galaxy

• mass of the interstellar matter is about 15% of the mass of the stars in this disk

• stars, gas and dust are not spread evenly throughout the disk but are concentrated into a central bar and a series of spiral arms

• central bar composed of mostly old yellow-red stars

• two main spiral arms appear to connect with the ends of the bar (blue light because they contain young, hot stars)

• the Sun is located about halfway between the center of the Galaxy and the edge of the disk, and only about 70 light years above its central plane

thin disk of young stellar matter

• think disk of young stars, gas and dust is embedded in a thicker but more diffuse disk of older stars

  • thicker disk extends about 1000 light years above and 1000 below the midplane of the thin disk, and contains only about 5% as much mass as the thin disk

• stars thin out with distance from the galactic plane

central bulge/nuclear bulge

• close to the galactic center (within about 10,000 light-years), the stars no longer confined to the disk, but form a central bulge

• bulge often obscured by dust, therefore hard to observe and determine its shape

  • shaped like a peanut

halo

• the thin and thick disks and the nuclear bulge of our Galaxy are embedded in a spherical halo of very old, faint stars that extends to a distance of at least 150,000 light years from the galactic centre

• most of the globular clusters are also found in this halo

dark matter

• the invisible mass of the Milky Way that extends beyond the boundary of luminous stars to a distance of at least 200,000 light years from the centre of the Galaxy

• emits no light, and cannot be seen with any telescope

• composition is unkown

• can only be detected because of its gravitational effects on the motions of luminous matter that we can see

  • we know that this dark matter halo exists because of its effects on the orbits of distant star clusters and other dwarf galaxies that are associated with the Galaxy

characteristics of the Milky Way

arms of the Milky Way

• radio observations of the disk’s gaseous component indicate that the Galaxy has 2 major spiral arms that energy from the bar and several fainter arms and shorter spurs

• Sun is near the inner edge of a short arm/spur called the Orion-Cygnus Arm (10,000ly long)

formation of spiral structure

• the way objects turn around the center of the Galaxy follows Kepler’s third law

• objects farther from the center take longer to complete an orbit around the Galaxy

• differential rotation explains why so much of the material in the disk of the Milky Way is concentrated in elongated features that resemble spiral arms

differential galactic rotation

• objects further from the Galaxy center take longer to complete an orbit around the Galaxy than do those closer to the center

• stars in larger orbits in the galaxy trail behind those in smaller ones

Sun’s orbit around the centre of the Galaxy

• orbits the center of the Milky Way

• Sun’s orbit is nearly circular and lies in the Galaxy’s disk

• speed of the Sun in orbit is about 200lm/s, which means it takes us approx. 225 million years to go once around the center of the Galaxy (galactic year)

total mass of Galaxy

• at least 2 × 10^12 Msun

make up of dark matter

• only possibility that works is it is composed of exotic subatomic particles that have not been detected on Earth

centre of the Galaxy

• the very centre contains a black hole with the mass equivalent to 4.6 million Suns

• all this mass fits within a sphere that has less than the diameter of Mercury’s orbit

• called a supermassive black hole

  • indicates that the mass they contain is far greater than that of the typical black hole created by the death of a single star

  • can grow by swallowing clouds of gas and nearby stars, and merging with other black holes

  • at the current rate, we observe clouds of gas and dust falling into the galactic center at the rate of about 1 Msun per thousand years

• currently no star formation at the galactic centre, but there is lots of dust and molecular gas that is revolving around the black hole, along with some ionised gas streamers that are heated by the hot stars

• Sagittarius A* lies at the centre

population I

• bright blue stars that are found in the spiral arms of galaxies

• only found in the disk and follow nearly circular orbits around the galactic centre

• e.g. bright supergiant stars, main-sequence stars of highs luminosity (O,B), members of young open star clusters

• interstellar matter and molecular clouds are found in the same places as population I stars

• wide range of ages

• heavy elements account for 1-4% of the total stellar mass

population II

• all stars in the halo and globular clusters of a galaxy

• shows no correlation with the location of the spiral arms

• found throughout the galaxy

• some are in disk, many follow eccentric elliptical orbits that carry them high above the galactic disk into the halo

• e.g. stars surrounded by planetary nebulae and RR Lyrae variable stars, stars in globular clusters

• consists of stars that formed very early in the history of the Galaxy, typical ages are 11-13 billion years

• less than 1/100th the concentration of heavy elements found in the Sun and sometimes lower

  • these stars were formed when the abundance of heavy elements was low

protogalactic cloud

• gave birth to our Galaxy

• assumed to be roughly spherical

• formation of Galaxy: cloud collapsed and formed a thin rotating disk

  • gravitational forces caused the gas in the thin disk to fragment into clouds or clumps with masses like those of star clusters

  • these individual clouds then fragmented further to form stars

  • since the oldest stars in the disk are nearly as old as the youngest stars in the halo, the collapse must have been rapid, requiring no more than a few hundred million years

tidal stream

• occurs when smaller galaxies pass close to the Milky Way, with the gravity of our Galaxy tugging on harder on the near side than the far side of the other galaxy

• the net effect is that the stars that originally belonged to the small galaxy are spread out into a long stream that orbits through the halo of the Milky Way

• can maintain its identity for billions of years