M31 (Andromeda Galaxy) Type: Spiral galaxy Sb Apparent magnitude: 4.4 Distance: 2.9 million ly M31 and the Milky Way are approaching each other at 100 km s -1 . M31 interacted with M32 200 million years ago, leading to a disturbance in the spiral structure of M31. It has two bright nuclei, this could be due to the engulfment of a smaller galaxy or the presence of a dust lane obscuring part of a single core.
Edwin Hubble In 1923 Edwin Hubble looked at galaxies (which he thought were nebulae) through a 100” reflector on Mt Wilson in the US. He took a photo of Andromeda.
Edwin Hubble Hubble detected Cepheid variables in other galaxies. He measured their periods and determined their distances. They were too far away to be part of our galaxy. His estimate for Andromeda was 750 000 ly, it’s actual value = 2 200 000 ly. Hubble didn’t realise there were two tyes of Cepheids: Population I and Population II . He assumed they were Population II but in 1952 Baade showed they were Population I - twice as luminous and therefore twice as further away. Period Luminosity Apparent Magnitude Distance
The Local Group The Large Magellanic Cloud (157 000 ly) and Small Magellanic Cloud (197 000 ly) are irregular dwarf galaxies visible in the southern hemisphere. They are satellite galaxies of the Milky Way. They are members of the Local Group consisting of 50+ galaxies including the Milky Way.
The Local Group The 3 largest galaxies in the Local Group are: Andromeda (1.5 x size of the Milky Way) Milky Way Triangulum galaxy (M33). Beyond the Local Group are other galaxies of varying shapes and containing a wide variety of objects. These were classified by Hubble in a ‘tuning fork’ diagram.
Hubble’s Classification of Galaxies
Hubble’s Classification of Galaxies Increasing E number - increasing eccentricity of ellipse. Sa - Sd classifies spirals according to the the tightness of their arms (Sa: large central bulge, tight arms; Sd: small bulge, loosely wound). SB galaxies have a bar-like structure running through the centre.
Galaxies Hubble’s classification of galaxies does not show the evolution of galaxies - this depends on the initial mass and the speed of rotation of the galaxy. Ellipticals : contain Population II and old Population I stars. Star formation ended a long time ago, very little interstellar dust. They are red in colour. They have a wide range of sizes and masses. Spirals : contain young Population I stars in the arms and older Population II stars in the nucleus. The spiral arms are formed by a pressure wave; regions of density greater than the average density are created activating star formation which are sometimes visible as bright blue knots. In barred spiral galaxies the arms originate at the ends of a bar running through the nucleus. Irregulars : contain mostly Population I stars. Most are a result of tidal interaction with another galaxy or collision. The LMC and SMC are sites of active star formation due to tidal forces by the Milky Way compressing regions of gas and dust in these galaxies.
Superclusters Our Local Group is an irregular cluster of galaxies. The Virgo cluster is also irregular and much larger than the Local Group with thousands of members. The nearest regular cluster is the Coma Cluster 300 million ly away. The core is dominated by 2 giant ellipticals surrounded by medium-sized galaxies. Dozens of clusters of galaxies group together to form superclusters which stretch up to 100 million ly across space. Coma Cluster
Redshift and the Universe To measure the distances of very distant objects (can only use Cepheids up to tens of millions of ly), we use the Doppler effect. Towards us (blueshift) Away from us (redshift) Doppler Effect: ( - 0 )/ 0 = v/c = Doppler shifted wavelength 0 = stationary wavelength v = velocity of rotation c = speed of light
Redshift and the Universe Beyond the Local Group, all galaxies show redshifted spectra i.e. they are all moving away from us. Hubble showed that the more distant the galaxy was, the faster it was receding from us. H 0 = 70 km s -1 Mpc -1 The Hubble Law: v = H 0 d v = recessional velocity of galaxy H 0 = Hubble constant d = distance to galaxy
Redshift and the Universe
Example: A line in the spectrum of a galaxy is red-shifted by 60 Å
(Angstrom = 10 -10 m), it’s rest wavelength is 6000 Å. How fast is the galaxy moving and what is it’s distance from us?
- 0 = 60 Å, = 6000 Å
60/6000 = v/(3 x 10 8 )
v = 300 000 m s -1 = 300 km s -1
D = v/H 0 = 300/70 = 4.29 Mpc = 4.29 million pc
1 pc = 3.26 ly
So distance of galaxy = 14 million ly
Ultraluminous distant objects In 1963 Schmidt (at Caltech) measured a huge redshift in the spectrum of 3C 273 , an unusually strong radio source. He calculated a recessional velocity of 0.15c (45 000 km s -1 ), leading to a distance of 2 billion ly. It is too far away to be a star and have such a high luminosity, it is a quasar, a relic of the early universe (there are no nearby quasars). 3C 273 with jet Chandra X-ray image
Active Galactic Nuclei Some galaxies possess active galactic nuclei (AGN). These are energetic centres which consist of a black hole. As gases fall into the black hole their gravitational energy is transferred into radiation, thus powering the nucleus (they are the central engine of AGN). Examples of AGN are Quasars, Seyfert galaxies and radio galaxies.
Quasars Quasars (Quasi-stellar radio sources) are the ultraluminous centres of distant galaxies. They are typically a thousand times more luminous than the Milky Way. They are among the most distant objects ever seen. IRAS 04505-2958 3 billion ly away
Seyfert and Radio galaxies Seyfert galaxies show strong emission lines as seen by Carl Seyfert at the Mt Wilson Observatory in 1943. The brightest Seyferts are as luminous as faint quasars. They have weak radio emissions. Some are interacting with other galaxies. Radio galaxies in contrast to Seyferts exhibit strong radio emission. They generally consist of 2 radio lobes on either side of the host galaxy which is usually a giant elliptical. They are sometimes called double radio sources. Cygnus A Radio lobes 160 000 ly from galaxy
Leo Regulus Denebola
M65 Type: Spiral galaxy Sa Apparent magnitude: 10.25 Distance: 35 million ly M65 forms a triplet with M66 and NGC 3628, two other spiral galaxies. It has tightly wound spiral arms and a dust lane. The galaxy has an old stellar population; near the dust lane are knots of gas representing star forming regions.
M66 Type: Spiral galaxy Sb Apparent magnitude: 8.9 Distance: 35 million ly M66 shows strong deformations through gravitational interactions with other galaxies (NGC 3628). The western spiral arm is separated and rising above the main galaxy. Star-forming regions are dominant in the northern parts.
M95 Type: Spiral galaxy SBb Apparent magnitude: 11.4 Distance: 38 million ly M95 is a barred spiral galaxy. The centre of the galaxy contains a circumnuclear (ring-shaped) star forming region of diameter 2000 ly. Image of galaxy in infrared light
M105 Type: Elliptical galaxy E1 Apparent magnitude: 10.2 Distance: 38 million ly The central region of M105 contains an object of mass 50 million times the mass of the Sun - this is thought to be a supermassive black hole.
M49 Type: Elliptical galaxy E4, Seyfert 2 Apparent magnitude: 9.4 Distance: 60 million ly M49 contains about 6300 globular clusters of stars.
M84 Type: Elliptical galaxy E1/S0, Seyfert 2 Apparent magnitude: 10.1 Distance: 60 million ly M84 is a giant lenticular/elliptical galaxy which is orientated face-on towards us. It has an active galactic nucleus which ejects jets of material in opposite directions.
M89 Type: Elliptical galaxy E0 Apparent magnitude: 10.7 Distance: 60 million ly M89 is an almost spherical galaxy that has a jet of material extending 100 000 ly outwards. This may be a smaller galaxy caught in the tidal gravitational forces of M89. It may once have been an active quasar or radio galaxy. It also has a large population of globular clusters, 10 times as many as the Milky Way.
M90 Type: Spiral galaxy Sab, Seyfert 2 Apparent magnitude: 10.3 Distance: 60 million ly M90 is a fossil spiral galaxy - there is no current star formation except in the inner disc region (where there are dark dust lanes). It may be evolving to type Sb and then onto a lenticular type. Multiple supernovae in the nucleus have produced fast winds that are blowing the galaxy’s interstellar medium outwards. M90 is moving towards the Earth (spectrum is blueshifted) in contrast to most other galaxies that are moving away from us (their light is redshifted).
M104 (Sombrero Galaxy) Type: Spiral galaxy Sa Apparent magnitude: 8.98 Distance: 30 million ly M104 has a bright core and a pronounced bulge with a richly populated globular cluster system. The bulge contains a supermassive black hole (it is an AGN). It has a prominent dust lane in its inclined disc. This dust ring is the primary site of star formation within the galaxy.
Canes Venatici Cor Caroli
M51 (Whirlpool Galaxy) Type: Spiral galaxy Sc, Seyfert 2 Apparent magnitude: 9 Distance: 37 million ly M51 has a pronounced spiral structure which is a result of a collision with its neighbour, NGC 5195. Gas in the galaxy was compressed in some regions, initiating star formation. These show up as bright blue knots in the spiral arms. M51 has a compact nucleus containing a black hole surrounded by a ring (torus) of dust. Black hole at centre of galaxy
NGC 6240 Type: Ultra-luminous Infrared Galaxy (ULIRG) Apparent magnitude: 12.8 Distance: 400 million ly Luminosity: 10 12 L S NGC 6242 is the remnant of a merger between two smaller galaxies. It has two distinct nuclei and a disturbed irregular structure. It is very bright in the infrared due to the large quantities of dust it contains.
Ursa Major Alkaid
M81 Type: Spiral galaxy Sb Apparent magnitude: 7.89 Distance: 12 million ly M81 has an active galactic nucleus (AGN). There was a close encounter between M81 and M82 600 million years ago which lasted 100 million years. M81 being much larger than M82 deformed M82. The distance between the centres of the galaxies is only 150 000 ly. The percentage of dark matter in M81 is lower than average. M82 is irregular and has heavy star formation and dark dusty lanes. It contains many young globular clusters as a result of the collision with M81. M82 Type: Starburst galaxy I Apparent magnitude: 9.3 Distance: 12 million ly
M108 Type: Spiral galaxy Sc Apparent magnitude: 10.7 Distance: 45 million ly M108 is an edge-on spiral. It has no bulge or pronounced core. M108 contains young star clusters and consequently is very dusty.
Ursa Minor Ursa Minor Ursa Major Polaris
Ursa Minor Dwarf Galaxy Type: Elliptical Galaxy Apparent magnitude: 11.9 Distance: 200 000 ly The Ursa Minor Dwarf Galaxy is a satellite galaxy to the Milky Way. It consists mainly of older stars with no active star formation.
Spectral Type Stars are classified according to their temperature and corresponding colour. They are ordered in this way: O (hottest, brightest stars - blue/white in colour), B, A, F, G, K, M (coolest, dimmest stars - red in colour) Apparent Magnitude A measure of the brightness of a star relative to other stars in the sky (doesn’t take into account intrinsic luminosity and distance of the star) Absolute Magnitude The apparent magnitude of the star if situated 32.6 ly away from the Earth. Light-years (ly) One light-year is the distance light travels in one year. This is equal to 9.46 x 10 12 km, or 9.5 million million km. L S, M S, R S These are the luminosities, masses and radii of the stars relative to the Sun. For example, if a star has a luminosity = 10L S , it’s luminosity is ten times that of the Sun. Astronomical Terms