Star Clusters and The Milky Way

Physics 360/Geol 360

Spiral Galaxy NGC5236 much like oursA typical globular cluster

The Milky Way Appears as a Band

HNSKY view of the sky tonight. Can you identify the Milky Way?

Early Estimates of the Milky Way

Size estimated in early 1900’s

Kapteyn (Dutch) About 20 kiloparsecs (kpc)

Shapley (US) About 100 kpc

Both were partly right and partly wrong—they misinterpreted the dimming effects of interstellar dust. Shapley placed the Sun’s position about right; Kapteyn incorrectly placed the Sun in the center.

Shapley measured the galaxy size with RR Lyrae stars.

Sketch of the Milky Way

Mass ~ 1011 mass of Sun About 1012 stars

Population II

Population I

Population IPopulation I and II

Early Estimates of Milky Way (MW) Size

• Review of Distances

– 1 parsec (pc) = 3.26 light years (ly) = 3 x 1013 km

– 1 kiloparsec (kpc) = 1000 parsecs = 3260 ly

• Early Distance Measurements of open and globular clusters were inaccurate due to dark matter and Hydrogen gas affecting the Luminosity Law. This gas and dust makes up to 15% of the MW mass

• The Milky Way consists of

– A disk of about 30 kpc (100,000 ly)

– A surrounding halo—dark matter

– A flattened bulge of stars surrounding the core

• Our Solar System lies 8.5 kpc from the center (28,000 ly) with orbits tilted 60o w/r to the galaxy disk (the Milky Way band is tilted w/r to the ecliptic)

The Milky Way – Our Galaxy

Many ancients observed a milky band of stars in the night sky running through the constellations of Cassiopeia, Perseus, Taurus, Monoceros, Vela, Crux, Norma, Sagittarius, Scutum, Aguila, Cygnus, and Lacerta.

Although spaces between the stars consist of a vacuum as good as can be gotten on earth (about 10-11 Torr or mm of mercury) there is still plenty of gas and dust called interstellar medium. About 99% of this medium is gas and the rest is dust.As to the composition of this dust “The dust is made of thin, highly flattened flakes or needles of graphite (carbon) and silicates (rock-like minerals) coated with water ice. Each dust flake is roughly the size of the wavelength of blue light or smaller. The dust is probably formed in the cool outer layers of red giant stars and

dispersed in the red giant winds and planetary nebulae. “ from from Nick Strobel's Astronomy Notes at http://astronomynotes.com/ismnotes/s2.htm

The Effect of Interstellar Dust

Light passing through interstellar dust suffers extinction (dimming of all wavelength of light) and scattering. The amount of the latter depends on the color of the light.

1. This page was copied from Nick

Strobel's Astronomy Notes. Go to his site at

www.astronomynotes.com for the updated

and corrected version.

Extinction and Scattering continued and early Milky Way estimates

In 1930 R.J. Trumpler discovered dust as he plotted the angular diameter of star clusters versus the distance to the clusters. He derived the latter from the inverse square law of brightness. After analyzing the data he concluded that more distant clusters simply have more “stuff” between us and the clusters hence they appear fainter than they really are. Trumpler showed that there is dust material between the stars and that the extinction of starlight is caused by the scattering of light out of the line of sight. This caused early observers to (although they recognized the disk shape of the milky way by observing star clusters) overestimate the diameter of the milky way. From the law of scattering bluer wavelengths are scattered more than redder wavelengths.

“The presence of interstellar gas can be seen when you look at the spectral lines of a binary star system. Among the broad lines that shift as the two stars orbit each other, you see narrow lines that do not move. The narrow lines are from much colder gas in the interstellar medium between us and the binary system. “ from Nick Strobel’s Astronomy Notes.

As was mentioned previously 99% of the material between the stars is gas and about 90% of the gas is atomic or molecular form of gas with the remaining 10% Helium plus trace elements.

Remember the dust has more of an effect on the visible wavelengths. Ionized hydrogen emits light in the visible band but neutral hydrogen and molecular hydrogen emit in the radio band.

The Anglo-Australian Observatory at http://www.aao.gov.au/images/ is worth looking at!

Nick’s Astronomy Notes offers the following explanation for H II regions

“H II regions are regions of hot (several thousand K), thin hydrogen emission nebulae that glow from the fluorescence of hydrogen atoms. The roman numeral ``II'' of H II means that hydrogen is missing one electron. A He III nebula is made of helium gas with two missing electrons. A H I nebula is made of neutral atomic hydrogen. Ultraviolet light from hot O and B stars ionizes the surrounding hydrogen gas. When the electrons recombine with the protons, they emit light mostly at visible wavelengths, and primarily at a wavelength of 656.3 nanometers (giving the hydrogen emission nebulae their characteristic red color). In this conversion of the ultraviolet energy, each ultraviolet photon produces a visible photon. The temperature of the stars causing the nebula to fluoresce can be estimated from this even though the O and B stars are hidden inside the nebula. Fluorescent light bulbs operate on the same basic principle except they use mercury vapor to produce ultraviolet light. The ultraviolet light is then converted to visible light by the phosphor layer on the inside of the glass bulb.”

The Orion Nebula -- It is the fuzzy patch you can see in the sword part of the Orion constellation

Explanation of previous slide from the Anglo-Australian Observatory

Stellar Population Types in our Milky Way Galaxy

• All stars are mainly Hydrogen and Helium

• Population I Stars– 2% elements other than Hydrogen and Helium– All spectral types

• Population II Stars – < 0.1% elements other than H and He– Only cooler spectral types (G, K, M)

Age of our Milky Way

• Our galaxy’s most ancient stars ~ 15 Billion Years Old

• (however another guess is 11 – 12 Billion)

• Blue Stars (Pop I) in disk and spiral arms

• Red Stars (Pop II) in bulge and halo

• This is true for other galaxies as well

• Pop II stars may have elongated tilted orbits whereas Pop I stars orbits are in the disk

The Forces of Gravity Shape our Milky Way• The center of our MW is probably a dense swarm of gas and stars and

a massive black hole which probably grows more massive by drawing in interstellar gas. Black holes such as these can grow even as large as 106 in a billion years.

• The flattened shape of a spiral galaxy such as ours implies that it rotates. Our Sun and nearby stars move around the MW with speeds of 220 km/sec and the disk and the MW disk (near us) makes one complete rotation in approx. 240 million years.

• What force keeps the galaxy together? – The collective gravitational attraction of the stars and gas within the MW draws them toward the center.

• The density of stars in the MW galaxy varies. Near the sun it is about 0.003 stars per cubic ly but near the core about 10 million stars per cubic ly or ly3.

• Near the edge stars are even spread more thinly than around our Sun.• On the far edge exists a Halo of Dark Matter.

How we determine the mass of the Milky Way

From the “modified” form of Kepler’s Third Law

Where m = mass of sun, M = mass of galaxy, a = radius of sun’s orbit around galactic mass, P = years to complete one orbit

Define a = 1.8 x 109 AU, p = 240 million years

M = 1011 M

From Thomas T. Arny, Explorations an introduction to astronomy

Finding distance to the center of the milky way using globular clusters

“Finding the orbital speed of the sun around the Milky way. Astronomers take spectra to obtain the Doppler shift of galaxies in the local group, the small cluster of galaxies to which the Milky Way belongs. The Doppler shift of the galaxies is created by their own motion and that of the Sun….It turns out that the galaxies move slowly compared with the Sun’s rotation around the Milky way so almost all the Doppler shift is attributable to the Sun’s motion.” Reference Arny, page 469

Star Clusters

• Open Clusters– Few hundred stars 7-20 ly across– All spectral types– Population I Stars

• Globular Clusters– Few million stars ~100 ly across– G stars and cooler– Population II stars

Star Clusters

Globular Cluster HR Diagrams

• Assume globular cluster stars formed together

• High-mass stars die first

• Age of cluster comes from mass of largest star left

• What is the age of this cluster?

Distant galaxies avoid the Milky Way disk –why?

Globular clusters show a distant galactic center

IR Picture of the Milky Way

IR travels through dust

This ‘all-sky’ picture clearly shows that we are at an edge of the galaxy

Improving View of the Milky Way

Improving View of the Milky Way

• Stars appear to occupy a disk centered on the Earth• Globular clusters occupy a sphere centered 20,000

ly away• We assume globular clusters are centered on the

galaxy• Dust in the disk prevents us from seeing the whole

disk• IR and radio observations confirm this view

Mapping Spiral Arms

Spiral arms are mapped by star-formingregion markers

Radio waves travel through dust and allow maps over the whole disk

Spiral ArmsAreas of Star Formation

• Long-life stars move on

• High mass stars die in arm

• Spiral Arms are bright– Gaseous nebula– Hot stars– Supernova

• Higher density -> gas collapse -> star formation

Inner Galaxy / Core

• The inner galaxy is a bar of stars and gas• The inner core contains a black hole

Dark Matter

• Star’s near the edge of the galaxy should orbit slowly

• They orbit quickly• Since they aren’t flying

into deep space, there must be extra gravity

• We don’t SEE a source Dark Matter!

Dark Matter

• Mass about 10 times the “luminous mass” (stars and nebula)

• What might is be?

• The nature of dark matter is one of the great problems in astronomy

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