Stellar halo
From NYU CCPP Wiki
by Kathy Zhang (NYU)
Contents |
definitions
Absolute magnitude
Absolute magnitude is the brightness of a star if it were viewed from a distance of 10 parsecs.
Apparent and absolute magnitude are related by the equation:
apparent magnitude - absolute magnitude = 5log10(distance/10 pc).
Color index
Photometric filter bands allow only a certain range of wavelengths above and below a central (or effective wavelength) to enter the telescope. The SDSS system uses bands u, g, r, i and z with effective wavelengths 352, 480, 625, 769, and 911 nm. A color index (or color) is the difference between the apparent magnitudes of a star viewed with different filter bands, and quantifies a star's color. For example using the standard UBVRI photometric system, the color B(bluelight) - V(visible/green-yellow light) is smaller for bluer stars and greater for redder stars. An accurate measurement of star temperature requires an examination of its spectra. However, for faint stars or surveys involving many many stars such as SDSS it is more practical to observe color which corresponds to the star's gross blackbody curve. Temperature from a blackbody curve is found using Wein's Law:
wavelength of peak emission from a star = 2.9*106nm*Kelvin / temperature.
Intensity
The intensity of radiation is the average amount of energy flowing per unit area per unit time at some point in space. Intensity is proportional to apparent brightness.
Luminosity
Luminosity is the total energy output of a star per unit time. Luminosity is proportional to absolute brightness. Therefore, 2.512M-Msun = Bsun/B = Lsun/L and:
absolute magnitude = 4.83 + 2.5 log10(Lsun/L) where 4.83 is the absolute magnitude of the sun.
Magnitude
Apparent brightness is proportional to luminosity / distance2. One measurement of apparent brightness is magnitude or apparent magnitude. Vega is defined to have a magnitude of 0.0, and a star of magnitude 5 is by definition 100 times fainter than Vega. It follows that a star is 2.512(difference in magnitude) times brighter or fainter than some other star. Stars of magnitude 6 are the faintest stars visible by the naked eye.
Metallicity
Metallicity is the percent of metal, that is elements heavier than hydrogen or helium, in a star, often expressed as [Fe/H] which is the log of the ratio of iron to hydrogen in a star to that in the sun: [Fe/H] = log10 [(NFe/NH)star/(NFe/NH)sun] or
[Fe/H] = log10 (NFe/NH)star - log10 (NFe/NH)sun ; N = particles/m^3.
A star with [Fe/H] = 1 contains 10 times as much metal than the sun, a star with [Fe/H] = 2 comtains 100 times as much metal than the sun, etc.
Proper motion
Proper motion is the angular rate of change in position of a star in the celestial sphere. Proper motion is inversely proportional to a star's distance, and from it one can determine a star's transverse velocity.
Radiation
Electric and magnetic fields influence one another and can form a self sustaining electromagnetic wave that propagates through space. Because microscopic charged particles are in constant motion, all macroscopic objects emit electromagnetic waves or radiation. These waves carry energy through space.
SDSS
The Sloan Digital Sky Survey (SDSS) is the most ambitious survey of the skies to date and when complete will have imaged a quarter of the sky in five colors, determined the position and absolute magnitude of 100 million celestial objects and taken spectra of 1 million objects. See www.sdss.org.
Spectral classes
There are ten spectral classes O, B, A, F, G, K, M, R, N, and S in order of decreasing surface temperature. Each have 10 sub classes, 0 - 9. The classes were originally labeled in alphabetical order such that A showed the strongest hydrogen absorption lines, B showed the strongest Helium lines, and the rest were ordered somewhat arbitrarily. Now we know that the state of ionization of a gas determines what spectral lines it forms, and because hotter gases are more highly ionized the spectral classes can be arranged in temperature order.
types of stars
Main Sequence stars
Stars that burn hydrogen into helium in their core are found on the main sequence of the Hertzsprung-Russell (H-R) diagram. The trend of main sequence stars is more luminous stars have hotter surface temperatures, and less luminous stars have cooler surfaces.
Red Giants
When a star with 0.08 - 8 times the mass of the sun uses up its core hydrogen, the core shrinks under the star's gravity thereby increasing the temperature of overlying layers and causing hydrogen there to fuse rapidly. The outer layers are pushed outward and the star is now a red giant.
Planetary Nebulae
Eventually the core of a red giant becomes so hot that helium fusion occurs, and the ultraviolet radiation the core emmits ionizes the inner parts of the escaping outer layers and makes it look really cool.
White Dwarfs
About 50,000 years after a star has become a red giant, the outer layers of the star drift away and what is left behind is a white dwarf, a core of carbon and oxygen about the size of the earth with a mass about half that of the sun (the average density of a white dwarf = 109 kg/m3). Neon-oxygen white dwarfs also exist and are the remnants of higher mass stars.
Red Supergiants
When a star with more than 8 solar masses dies it will become much bigger after becoming a red giant. An 8 - 12 solar mass star will leave behind a neon-oxygen white dwarf, and stars with greater than 12 solar masses will eventually explode as a supernova, leaving behind a neutron star or black hole.
Blue Supergiants
A red supergiant evolved from a 15+ solar mass main sequence star steadily loses its extended atmosphere due to strong stellar winds, becoming the smaller and hotter blue supergiant. Blue supergiants of spectral types B and A emit almost all their light in the visual spectrum and are the visually brightest stars.
Neutron Stars
A star with 1.4 to 4 times the mass of the sun remaining after a supernova explosion will collapse into a neutron star. The average density of a neutron star can reach 1018 kg/m3.
Black Holes
A star more massive than 4 suns remaining after a supernova will collapse into a black hole.
tracers
Proper-motion surveys
Because halo stars' orbits differ greatly from that of the sun they have large proper motions. However, proper-motion surveys can identify stars only up to a few hundred parsecs away.
RR Lyrae
The horizonal branch (HB) is an era of the stellar evolution of low mass stars (.08suns<mass<8suns) where helium is burning steadily in its core. Below you can see the HB of the globular cluster M55. The importance of this region is that pulstating stars called RR Lyrae variables are found on the HB of the metal poor globular clusters in the halo. [There is a region called the instability strip that slices through the H-R diagram in which variable stars are observed. It happens that main sequences of more metal poor populations shift down and to the left resulting in the intersection of halo globular clusters' HB and the instability strip.] RR Lyrae variables have periods of 0.2 to 2 days and amplitudes of 0.3 to 2 magnitudes, and all have the same average absolute magnitude. Knowing absolute and apparent magnitude we can determine the distance to any identified RR Lyrae star including those found outside of a cluster. The density of RR Lyraes in the sky is one per square degree.
Blue Horizontal Branch
Blue horizontal branch (BHB) stars are those found on the HB to the left, blue side of RR Lyrae variables. BHB stars are very good tracers because they are extremely bright and most faint blue stars seen in the halo are usually BHB stars. They are identified by their very blue color, and by a spectroscopic check of gravities to differentiate them from blue stragglers which have the same colors as BHB stars but higher gravities. Distance is determined in the same way for RR Lyrae stars since BHB stars have the same absolute magnitude as RR Lyraes. Also, one can determine the number of red horizontal branch (RHB) stars (and total number of HB stars) from the ratio of BHB to RR Lyrae stars and from that the density of all star types in a given region. BHB stars are about as rare as RR Lyraes.
Red Horizontal Branch
Red horizontal branch (RHB) stars are found to the right of RR Lyrae variables.
Blue stragglers
Blue stragglers are hot main sequence stars of a population that lie beyond the turnoff point. They are believed to have formed from the merging of stars.
Main-sequence turnoff stars
MS turnoff stars can be recognized by their color (0.38<B-V<0.5).
Carbon stars
Carbon stars are found at the tip of the asymptotic giant branch (AGB)--As a red giant gets older and nears the top of the AGB oscillations develop within the star which may cause carbon nuclei to be brought to its surface, and the giant becomes a carbon star. Carbon stars are peculiar red giants that have carbon to oxygen ratios four to five times higher than those of typical red giants. They are easily identified from their spectra, but are extremely rare (one per 200 square degrees).
milky way
Local group
The Milky Way is part of a small cluster of galaxies of about thirty members. Our cluster the Local Group is 800 kcp in diameter. Our closest neighbors the Large and Small Magellanic Clouds are 50 - 60 kpc from the sun. They are low mass type I irregular galaxies and are orbiting the Milky Way. Another galaxy, the spiral Andromeda lies 700 kpc from the sun and is the most distant object visible to the naked eye.
Observing the milky way
The most prominent feature of the Milky Way, the galactic plane or galactic equator as seen from Earth runs from the Northern Cross to the Southern Cross and wraps around the celestial sphere. The center of our galaxy is located on the constellation of Sagittarius, and the coordinate of the north galactic pole is (RA, dec) = (12h 49m, 27ยบ 24').
Structure
The Milky Way is a barred spiral galaxy, and consists of a disk, bulge and halo. The galaxy contains 200 billion stars, the most of which are found in the disk:
thin disk
- The disk is a plane of stars orbiting the galactic center and is 30 kcp in diameter. It consists of an inner thin disk and outer thick disk. The thin disk has a height of 90 - 325 pc. It contains a high density of interstellar gas, and is the site of ongoing star formation. Most of the luminous stars (spectral type O and B) in the thin disk are located in its spiral arms. There is the same density of stars in the gaps between the arms as there are within the arms. However, hydrogen gas is more concentrated in its spiral arms. The metallicities of stars in the thin disk range from 1/3 to 3 times that of the sun, and the sun is representative of the general metallicity of thin disk stars.
thick disk
- The thick disk has a height of 580 - 750 pc and contains 10 percent of the stellar mass of the Milky Way. The thick disk is entirely made up of stars. Like the halo most stars in the thick disk are more than 10 billion years old. The metallicities of stars in the thick disk range from 1/10 to 1/2 that of the sun and are typically 1/4 that of the sun.
bulge
- About the center of the galaxy lies a football shaped bulge with dimensions along the plane of the disk 6 kcp by 3 kcp and perpendicular to the disk a height of 4 kcp. The bulge is dominated by stars older than 7 billion years old but also contains younger stars that are 5 billion to less than 500 million years old. Although the majority of bulge stars have similar ages to those of the halo, bulge stars are more metal rich.
galactic center
- The high speeds of interstellar clouds near the nucleus of the bulge and the enormous amounts of X rays being emitted implies that the galactic center known as Sagittarius A is a black hole of 2.5 million solar masses.
halo
- The halo, a large and stellar diffuse region surrounds the disk and bulge--Only 1% of the galaxy's stellar mass resides in the halo. Of that 1%, 99% is contributed by field stars and 1% by globular clusters. There are 146 globular clusters distributed spherically inside the inner halo which extends 20 kcp in all direction from the galactic center. The globular clusters can be divided into two systems, the metal rich ([Fe/H] >= -0.8) "disk" globular cluster and the metal poor ([Fe/H] < -0.8) halo globular clusters or F clusters. The distribution of the disk globular clusters is more flattened, and disk clusters are fewer in number than F clusters.Field stars are those not bound in a cluster. Half of all known field stars are members of a binary systems.
The boundary of the extended halo lies several kpc above and below the disk and has a diameter of roughly 100 kpc. Radio observations have shown that there is an unexpectedly high amount of mass beyond the outer portions of the disk, and as much as 90 percent of the galaxy's total mass may be in the extended halo in the form of dark matter.
There is little interstellar material in the galaxy's halo and star formation has not occured there for the last 10 billion years. Accordingly halo stars contain a relatively low abundance of heavy elements. Generally the ratio of heavy elements in halo stars to the sun is 1/100.
Orbital motion
disk - In the disk of the galaxy stars follow Keplerian orbital motion save for the innermost region in which stars held together by their combined gravitational forces orbit like a rigid object. Our sun lies 8 kpc (about two thirds of the way) out from the galactic center in the disk and has an orbital velocity of 220 km/s. It takes about 250 million years for the sun to complete its orbit.
bulge and halo - Orbits in the bulge and halo appear random and are not confined to a plane but move in three dimensions. However, at any given distance from the galactic center, halo stars move at speeds comparable to the disk's rotational speed at the same radius.
