A giant • It has a mass between 750 billion and one trillion solar masses, • And its diameter is about 100,000 light years
Spiral Galaxy • Concluded that it is spiral through the use of radio astronomy and observations of other galaxies • Means that it has pronounced disk component exhibiting a spiral structure, and a prominent nuclear reagion which is part of a notable bulge/halo component
Belongs to the Local Group • Includes our Milky Way Galaxy, the Large and the Small Magellanic Cloud (LMC and SMC), Andromeda Galaxy also known as M31 and over 30 small galaxies, • It is the second largest (after the Andromeda Galaxy M31)
These galaxies are spread in a volume of nearly 10 million light years diameter, • It is centered somewhere between the Milky Way and M31. • Membership is not certain for all these galaxies, and there are possible other candidate members.
the Milky Way and M31 are by for the most massive, and therefore dominant members. Each of these two giant spirals has accumulated a system of satellite galaxies, where • the system of the Milky Way contains many (nearby) dwarf galaxies, spread all over the sky, namely LMC, SMC, and the dwarf galaxies in Ursa Minor, Draco, Carina, Sextans (dwarf), Sculptor, Fornax, Leo I and Leo II, • the system of the Andromeda galaxy is seen from outside, and thus grouped around its main galaxy M31 in Andromeda, containing bright nearby M32 and M110 as well as fainter and more far-out NGCs 147 and 185, the very faint systems
Neighbors • The closest of all is SagDEG at about 80,000 light years from us and some 50,000 light years from the Galactic Center, followed by the more conspicuous Large and Small Magellanic Cloud at 179,000 and 210,000 light years, respectively
The spiral arms of our Milky Way contain interstellar matter, diffuse nebulae, and young stars and open star clusters emerging from this matter. • The bulge component consists of old stars and contains the globular star clusters; • our galaxy has probably about 200 globulars, of which we know about 150. • These globular clusters are strongly concentrated toward the Galactic Center
From the apparent distribution of clusters in the sky, Harlow Shapley concluded that the center of the Milky Way lies at a considerable distance (which he overestimated) in the direction of Sagittarius and not rather close to us, as had been thought previously by Kapetayn (this is our next lab calculating the distance to the galactic center)
Our solar system is thus situated within the outer regions of this galaxy, well within the disk and only about 20 light years above the equatorial symmetry plane but about 28,000 light years from the Galactic Center. • It takes our Solar System 225 million years or so to revolve once around the galactic center
Tracing the Milky Way's history • The oldest stars (in the globular clusters) orbit the galaxy in elongated elliptical orbits. • Galaxy must have been born from huge gas cloud • Where are the first stars without heavy metals?
History • believe that the galaxy formed out of a large, fairly spherical cloud of cold gas, rotating slowly in space. • At some point in time, the cloud began to collapse in on itself, or condense • Similar to how the solar system and stars formed • Initially, some stars may have formed as the gas cloud began to fragment around the edges
condensing further to form a star or group of stars. • Some very old stars are distributed in a spherical halo around the outside of the galaxy today because the cloud was spherical in the beginning. • At such early times, these stars consisted only of the hydrogen and helium gas which made up the cloud.
The cloud continued to collapse, with more and more stars being formed as it did so. Since the cloud was rotating, the spherical shape began to flatten out into a disc, and the stars which were formed at this time filled the disc regions. • Identical to the process of forming a solar system
As the formation of new stars continued, some stars evolved through their “life” or died • These stars began to shed their atmospheres or explode in huge supernova events. • Through this process older citizens of the still young galaxy enriched the gas in the cloud with the new, heavier elements which they had formed, • the new stars being created in the disc regions contained the heavier elements. • Astronomers call these younger, enriched stars population 1 stars, and the older stars population 2.
The solar system is situated within a smaller spiral arm, called the Local or Orion Arm, which is merely connection between the inner and outer next more massive arms, the Sagittarius Arm and the Perseus Arm
A Puzzle of the Arm • Why don't they wrap around the MW? 225,000,000 years for the Sun to circle the Milky Way age of sun is about 4,500,000,000 years • Therefore - the sun has been around the Milky Way 20 times • If the arms had been around this many times they would have wound up. • But - arms are formed by new stars (which fade away in 107-108 years)
Answer • Sun takes 225,000,000 years to orbitArms fade away (O & B stars) in about 10,000,000 years and new arms form
Theories of Arm Formation • Density wave theory- Density waves move through galaxy compressing matter as it passes and setting off star formation. • self-propagating star formation (or Supernovae chain reaction)- One supernova sends out wave which creates new stars and subsequent supernovae.
The Milky Way system is a spiral galaxy consisting of over 400 billion stars, plus gas and dust arranged into three general components as shown to the left: • The halo - a roughly spherical distribution which contains the oldest stars in the Galaxy, • The nuclear bulge and Galactic Center. • The disk, which contains the majority of the stars, including the sun, and virtually all of the gas and dust
Halo • consists of the oldest stars known, including about 146 Globular Clusters, believed to have been formed during the early formation of the Galaxy with ages of 10-15 billion years from their H-R Diagrams. • The halo is also filled with a very diffuse, hot, highly-ionized gas. The very hot gas in the halo produces a gamma-ray halo.
Dark Halo • Radius of 200,000 or 300,000 light years • Contains 80%-95% of the mass of the galaxy (dark matter)
Not much is known about the mass of the halo. • Investigations of the gaseous halos of other spiral galaxies show that the gas in the halo extends much further than previously thought, out to hundreds of thousands of light years. • Studies of the rotation of the Milky Way show that the halo dominates the mass of the galaxy, but the material is not visible, it it thought to be dark matter.
Galactic Bulge • Radius of 6,000 light years • Composed of old stars (population II) and young stars (population I)
Radius of Central Bulge • 6,000 light years • Thickness of M.W. Disk • 1,000 light years
Galactic Disk • Radius of 60,000 light years • old stars (population II) and young stars (population I), gas, and dust • Spiral arms
Galactic Halo • Radius of 65,000 light years • Includes globular clusters • which contain only old stars (population 2)
Population I Stars • younger, metal-rich stars • found in galactic disk • Population II Stars • older, metal-poor stars • found in galactic halo
Stellar Parallax: • Angle through which a star's position shifts as earth orbits the sun. (actually this only works in determining stellar distances or nearby stars.) • Nearest Stars: Alpha Centauri complex (triple-star system) Proxima Centauri at 1.3 pc (4.3 ly) 0.76 arc-secs Barnard's Star 1.8 pc (6.0 ly) 0.55 arc-secs • Since stellar parallax fails beyond 100 parsecs or so, other methods need to be used1
Luminosity • If we know how bright something really is (known as Luminosity or Absolute Brightness) then we can determine from how bright it appears (Apparent Brightness) how far away it is • Calculated through Variable Stars measure distances out to about 15 million parsecs (15 Mpc)
Rotation • In the disk of the Milky Way, stars and other matter are rotating around the center in a regular pattern, as revealed by Doppler effects
Very important clue on evolution of galaxy The lower the metal abundance (pop I vs. pop II), the farther the objects are found from the plane of the disk (remember, low metal abundance (pop II) means old) stars are born with elemental abundance's of gases of birth stars inherit orbital motion abut galaxy of parent cloud massive stars evolve quickly and spew heavy elements into interstellar medium.
Evolution of MW Galaxy • Huge cloud of primordial gas collapsed into galaxy • Broke apart into small clouds -> formed stars • First stars (population II) formed in globular clusters more than 10 billion years ago. • Galaxy continued to Evolve • Sun emerged 5 billion years ago.