The Milky Way
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The Milky Way






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The Milky Way The Milky Way Presentation Transcript

  • The Milky Way
  • The Milky Way The Milky Way contains 100 000 million stars (1011). It is visible as a bright band across the sky (looking into the plane of the galaxy). The Sun is 30 000 ly away from the Galactic Centre, which lies in the direction of Sagittarius and is obscured by copious amounts of interstellar dust. There is a supermassive black hole in the centre. Sun
  • The Milky Way The spiral structure of the Milky Way was first proven by radio observations. Cold hydrogen gas emits radiation of wavelength 21 cm; plotting this radiation reveals spiral arms. The whole galaxy is rotating with a period of ~ 225 million years. 30 000 ly 20 000 ly Sun 100 000 ly Infrared map of the Milky Way
  • Open Clusters These are loosely packed groups of stars. The interstellar distances are large so collision is highly unlikely. Moving clusters are clusters of stars that move in a similar direction despite their large distances from one another. Five of the seven stars in Ursa Major make up the Ursa Major Moving Group. The two end stars, Dubhe and Alkaid are moving in the opposite direction. As a result the shape of Ursa Major will slowly change over time. M44 - Beehive Cluster
  • Globular Clusters These are large regular groups of stars. They may contain up to a million stars arranged in a spherical form. Interstellar distances are of the order of light-days and light- weeks, much closer than in an open cluster. The stars are old giant stars and so globular clusters are red in colour. Most globular clusters are very distant from us, only a few lie within 20 000 ly. Just over a 100 are visible in the Milky Way. NGC 5139 - Omega Centauri
  • Globular Clusters Globular clusters are located round the edges of the Milky Way. They contain short- period variable stars called RR Lyrae (period ~ 1 day). Their periods are related to their brightness so they can be used to estimate the distance of the globular cluster. An American astronomer called Harlow Shapley measured the distances of globular clusters and found they were not uniformly (evenly) distributed over the sky. They were concentrated in the southern hemisphere near the constellation of Sagittarius. This is in the direction of the galactic centre, away from the position of the Sun in the Milky Way.
  • Open Clusters vs Globular Clusters Open clusters consist of Population I type stars. These are very hot, young blue/white stars. Globular clusters contain Population II stars. These are old red giants and supergiants. IC 4665 - Open Cluster M56 - Globular Cluster
  • Taurus Pleiades Aldebaran
  • Hyades Type: Open cluster Apparent magnitude: 0.5 Distance: 151 ly Diameter: 65 ly Age: 625 million yrs The Hyades is the nearest open cluster to us. It is a spherical group of 300+ stars. The brightest stars in the cluster form a V-shape in the sky along with Aldebaran (which is independent of the cluster). The 4 brightest member stars are all red giants that started as A-type stars.
  • M45 (Pleiades/Seven Sisters) Type: Open cluster Distance: 440 ly Diameter: 86 ly Age: ~100 million yrs The Pleiades contain hot blue stars formed in the last 100 million yrs. The cluster will survive for another 250 million yrs after which it will have dispersed due to gravitational interactions with the stellar neighbourhood.The cluster contains many brown dwarfs (stars that failed to start hydrogen fusion), they make up about 25% of the population. Several white dwarfs have been seen, thought to be the products of high-mass stars in binary systems (not low-mass stars).
  • X-ray image M1 (Crab Nebula) Type: Supernova Remnant Apparent magnitude: 8.4 Distance: 6500 ly Diameter: 11 ly Mass of nebula: 4.6MS Temperature of nebula: 11 000 - 18 000 K Radius of central star: 30 km Age: ~1000 yrs The Crab Nebula has a pulsar at the centre - this is a high speed rotating neutron star, spinning at a rate of 30.2 times per second. In July 1054 Chinese astronomers recorded the supernova (of a star of mass 9 - 11MS). The resulting nebula has been expanding ever since, currently at a rate of 1500 km s-1. The nebula consists of filaments of the progenitor star’s atmosphere - ionised helium and hydrogen, carbon, oxygen, nitrogen and other elements. Strong magnetic fields create narrow beams of radiation from the neutron star.
  • Ursa Major β δ Alkaid
  • M97 (Owl Nebula) Type: Planetary Nebula Apparent magnitude: 9.9 Distance: 2600 ly Mass central star: 0.7MS Mass nebula: 0.15MS Age: 6000 yrs M97 is a cylindrical torus shell where the ends of the cylinder resemble the eyes of an owl. The shell is enveloped by a fainter nebula of lower ionisation.
  • Hercules η ζ β
  • M13 Type: Globular cluster Apparent magnitude: 5.8 Distance: 25 100 ly Diameter: 168 ly Age: 14 000 million yrs M13 contains 300 000 stars. Its brightest star has a magnitude of 11.95.
  • M92 Type: Globular cluster Apparent magnitude: 6.3 Distance: 26 000 ly
  • Orion
  • M42 (Orion Nebula) Type: Emission nebula Distance: 1270 ly Diameter: 24 ly M42 is one of the brightest nebulae in the sky. It is the closest region of massive star formation to the Earth. It contains a young open cluster of stars - the Trapezium cluster. There are four primary stars, two of which are binaries. They are part of a much larger cluster which contains 2000 stars. The Trapezium O-type stars ionise the surrounding gases. The red hue arises from ionised hydrogen, blue-violet areas are reflected radiation from the stars, the green regions arise from ionised oxygen. The star θ1 Orionis C is responsible for most of the ultraviolet radiation, creating a bright photoionisation front called the Orion Bar.
  • IC 434 (Horsehead Nebula) Type: Dark nebula Distance: 1500 ly IC 434 has a red glow from hydrogen gas ionised by σ Orionis. Streams of gas leaving the nebula are funnelled by a strong magnetic field. Stars are being formed in the base of the nebula.
  • M43 Type: Emission nebula Distance: 1600 ly M43 is an ionised hydrogen region, separated from the main nebula by a lane of dust.
  • M78 Type: Reflection nebula Distance: 1600 ly
  • Barnard’s Loop Type: Emission nebula Distance: 1600 ly Size: 300 ly Barnard’s loop is a nebula ionised by young stars. It originated from a supernova 2 million years ago.
  • NGC 2024 (Flame Nebula) Type: Emission nebula Distance: 900 ly NGC 2024 is ionised by ultraviolet radiation from ξ Ori, resulting in red emission from hydrogen.
  • Lyra Vega β
  • M56 Type: Globular cluster Apparent magnitude: 8.3 Distance: 32 900 ly Diameter: 85 ly M56 has a dozen observed variable stars of RV Tauri type (period = 90 days) and Cepheid type (1.51 days). The brightest stars in this loose globular cluster are of 13th magnitude.
  • M57 (Ring Nebula) Type: Planetary Nebula Apparent magnitude: 8.8 Distance: 2300 ly Mass central star: 1.2MS M57 is a bipolar nebula (not spherical!) illuminated by a hot white dwarf star in the centre. The blue-green colours are due to doubly-ionised oxygen emission (at 495.7 and 500.7 nm). The reddish outer areas are an indication of hydrogen emission (at 656.3 nm) and also emission from doubly-ionised nitrogen (at 654.8 and 658.3 nm). The nebula has been expanding for about 1600 yrs.
  • Canes Venatici β Cor Caroli
  • M3 Type: Globular cluster Apparent magnitude: 6.2 Distance: 34 000 ly Mass: 245 000MS Diameter: 180 ly Age: 5000 - 12 000 million yrs M3 contains 500 000 stars of which many are variable - of type RR Lyrae.
  • Hydra δ α
  • HST image showing white dwarf in the centre. NGC 3242 (Eye Nebula) Type: Planetary Nebula Apparent magnitude: 7.7 Distance: 2500 ly NGC 3242 is sometimes called the ‘Ghost of Jupiter’. The central star is a white dwarf.
  • Ophiuchiu s
  • IC 4665 Type: Open cluster Apparent magnitude: 8.27 Distance: 1400 ly
  • M12 Type: Globular cluster Apparent magnitude: 7.68 Distance: 16 000 ly Diameter: 74 ly M12 has lost about a million of its stars due to the gravitational influence of the Milky Way.
  • M62 Type: Globular cluster Apparent magnitude: 7.39 Distance: 23 000 ly Diameter: 90 ly M62 contains many variable stars, mostly RR Lyrae types. There are also X-ray sources such as close binary systems and pulsars that rotate in milliseconds.
  • Serpens β δ α
  • M5 Type: Globular cluster Apparent magnitude: 6.65 Distance: 25 000 ly Diameter: 160 ly Age: 13 billion yrs M5 is a very old cluster with up to 500 000 stars.
  • M16 (Eagle Nebula) Type: Open Cluster Apparent magnitude: 6 Distance: 25 000 ly Diameter: 140 x 110 ly Age: 5.5 million yrs M16 is a young cluster that is associated with the diffuse emission nebula (of ionised hydrogen), IC 4703. Active star formation is occurring in M16 in the dusty ‘pillars of creation’. The level of star formation peaked about a million yrs ago.
  • Vulpecula α
  • M27 (Dumbbell Nebula) Type: Planetary Nebula Apparent magnitude: 7.5 Distance: 1360 ly Mass central star: 0.56MS Radius central star: 0.055RS Age: 14 600 yrs M27 is expanding at a velocity of 31 km s-1. It contains knots of material, at the head of the knots are bright regions which represent local photoionisation fronts (radiation from the central white dwarf is ionising the ejected stellar material).
  • Auriga Capella β
  • M37 Type: Open cluster Apparent magnitude: 6.2 Distance: 4400 ly Diameter: 20 - 25 ly M37 contains over 500 stars including red giants. The hottest main sequence star in the cluster is of spectral type B9V.
  • Canis Major β δ
  • M41 Type: Open cluster Apparent magnitude: 4.5 Distance: 2300 ly Diameter: 25 ly Age: 190 - 240 million yrs M41 has over 100 stars and is moving away from us at 23 km s-1.
  • Capricornus β δ
  • M30 Type: Globular cluster Apparent magnitude: 7.7 Distance: 26 000 ly
  • Cassiopeia α β δ
  • M52 Type: Open cluster Apparent magnitude: 5 Distance: 5000 ly
  • Gemini Castor Pollux
  • M35 Type: Open cluster Distance: 2800 ly
  • Pegasus β α
  • M15 Type: Globular cluster Apparent magnitude: 6.2 Distance: 33 600 ly Diameter: 176 ly The core of M15 has collapsed and possibly formed a black hole. There are a high number of variable stars present in the cluster.
  • Perseus δ Algol
  • NGC 869 and NGC 884 Type: Open clusters Apparent magnitude: 5.3 and 6.1 Distance: 6800 and 7600 ly Age: 19 and 12.5 million yrs
  • M76 (Little Dumbbell Nebula) Type: Planetary Nebula Apparent magnitude: 10.1 Distance: 3400 ly Temperature central star: 60 000 K M76 resembles the Dumbbell Nebula M27 in Vulpecula.
  • NGC 1499 (California Nebula) Type: Emission Nebula Apparent magnitude: 6 Distance: 1000 ly NGC 1499 is mainly ionised hydrogen gas in a shape that resembles California.
  • NGC 1333 Type: Reflection Nebula Apparent magnitude: 5.6 Distance: 1000 ly NGC 1333 reflects light from nearby young large stars.
  • Sagittarius λ σ
  • M55 Type: Globular cluster Apparent magnitude: 7.42 Distance: 17 300 ly Diameter: 96 ly
  • M8 (Lagoon Nebula) Type: Emission Nebula Apparent magnitude: 6 Distance: 4100 ly Diameter: 110 x 40 ly M8 is an interstellar cloud, emission nebula and HII region. It has active star formation. M8 contains dark globules, these are collapsing clouds of protostellar material where stars are formed. The Hourglass Nebula is in the centre.
  • M17 (Swan Nebula) Type: Emission Nebula Apparent magnitude: 6 Distance: 5000 - 6000 ly Diameter: 15 ly M17 is a HII (ionised hydrogen) region. A cluster of 35 stars embedded in the nebula excite the gases giving rise to the beautiful colours.
  • M20 (Trifid Nebula) Type: Emission Nebula Apparent magnitude: 6.3 Distance: 2000 - 9000 ly M20 is a HII (ionised hydrogen) region. It is divided into three lobes by dark dust lanes (thus the name Trifid). It contains an open cluster, emission nebula (red area), a reflection nebula (blue region) and dark nebulae. It contains protostars and young stars; high energy stellar jets are the exhaust gases of this star formation.
  • Astronomical Terms 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 1012 km, or 9.5 million million km. LS, MS, RS These are the luminosities, masses and radii of the stars relative to the Sun. For example, if a star has a luminosity = 10LS, it’s luminosity is ten times that of the Sun.