Astronomy

Solar Spectral Types and Dwarf Stars

Solar Spectral Types and Dwarf Stars


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On the following page,

http://www.uni.edu/morgans/astro/course/Notes/section2/spectraltemps.html

It mentions Main, Giants and SuperGiants.

However for Dwarf Stars (e.g. D, Sd) , do I apply Main Sequence to the solar type?

For other types, IV, II, do I apply Main Sequence / Subgiant / Giant to the type?

If you look at the linked webpage, you'll see it mentions Types (V,III,I) but for instance where would Procyon (F5IV-V), Sargas (F1II) fit or should there be a different table for types IV, II?

Would groups (D, sd, IV,m II) have a different table? Rigil Kentaurus (G2V) would clear fit into Main Sequence.


The modern system of stellar classification is 2-dimensional, with one axis conveying spectral class, and the other conveying luminosity class. The spectral types are as follows:

  • O-type, "blue", $geq 30000$ $mathrm{K}$
  • B-type, "blue-white", $10000-30000$ $mathrm{K}$
  • A-type, "white", $7500-10000$ $mathrm{K}$
  • F-type, "yellow-white", $6000-7500$ $mathrm{K}$
  • G-type, "yellow", $5200-6000$ $mathrm{K}$
  • K-type, "orange", $3700-5200$ $mathrm{K}$
  • M-type, "red", $2400-3700$ $mathrm{K}$

There are also some extended spectral types, including D for white dwarfs, C for carbon stars, and several more for brown drawfs. The above are the predominant.

The magnitude axis consists of the following groupings:

  • 0-type, "hypergiants"
  • I-type, "supergiants"
  • II-type, "bright giants"
  • III-type, "giants"
  • IV-type, "subgiants"
  • V-type, "dwarfs" (these are the main-sequence stars)
  • VI-type, "subdwarfs"
  • VII-type, "white dwarfs"

When these systems are used, a spectral class is combined with a number ranging from 0-9 which symbolizes the location between adjacent spectral classes, and the luminosity class is then appended.

For example, the Sun is a type G2V star. This means it is a G-type star on the main sequence. It could then informally also be referred to as a "yellow dwarf".

The system is more complex than this, and there are many additional suffixes and other symbols to further explain the nuances of individual stars. This does cover the basics quite well, however.


Solar Spectral Types and Dwarf Stars - Astronomy

The relationship between mass and spectral class for main-sequence stars has never been obtained for dwarfs cooler than M6 currently, the true nature of objects classified as M7, M8, M9, or later (be they stellar or substellar) is not known. In this paper, spectral types for the components in five low mass binary systems are estimated based on previously published infrared speckle measurements, red/infrared photometry, and parallax data, together with newly acquired high signal-to-noise composite spectra of the systems and revised magnitude difference relations for M dwarfs. For two of these binaries, the secondary has a smaller mass (less than 0.09 solar mass) than any object having a dynamically measured mass and a known spectral type, thus extending the spectral class/mass relation to lower masses than has previously been possible. Data from the higher mass components (0.09 solar mass less than M less than 0.40 solar mass) are consistent with earlier results the two lowest mass objects -- though having mass errors which could place them on either side of the M dwarf/brown dwarf dividing line (Mass is about 0.08 solar mass) -- are found to have spectral types no cooler than M6.5 V. An extrapolation of the updated spectral class/mass relation to the hydrogen-burning limit suggests that objects of type M7 and later may be substellar. Direct confirmation of this awaits the discovery of a close, very late-type binary for which dynamical masses can be measured.


The "Fingerprints" of Stars

The best tool we have for studying a star's light is the star's spectrum. A spectrum (the plural is "spectra") of a star is like the star's fingerprint. Just like each person has unique fingerprints, each star has a unique spectrum. Spectra can be used to tell two stars apart, but spectra can also show what two stars have in common.

The spectrum of a star is similar to the spectrum of colors you see in rainbows. Stars give off light in a range of different colors. The spectrum of visible light - the spectrum you see in a rainbow - is shown below.

The wavelength of light determines its color. The wavelength on the spectrum above is measured in units called Angstroms 1 Angstrom = or 0.0000000001, or 1 x 10 -10 , meters.

Stars do not give off the same amount of light at every wavelength. If you made a rainbow graph like the one above for a star, some parts of the graph would be much brighter than others. Scientists used rainbow graphs for many years but in the past 20 years, they have begun to use an x-y graph to show a star's spectrum. The x-axis shows wavelength of light. The y-axis shows how bright the light is at that wavelength. Today, when scientists say "spectrum," they usually mean this x-y graph.

A typical spectrum for a star has a lot of peaks and valleys. You can see a typical star's spectrum below.

Click on the image to see it full size

Many of these peaks and valleys have labels on them. You may recognize some of these labels as symbols of chemical elements. Each star has a different set of peaks and valleys that can be used to divide the stars into different "spectral types."

The spectral types that astronomers use are given by the letters O,B,A,F,G,K,M (and there are some new spectral types that have been added in the last couple of years. more on those later!) For example, our sun is a type G star.

Before you find out what these letters mean, take a shot at developing your own system for classifying stars based on their spectra.


Illustration of Nearby Brown Dwarf

Observations of a nearby brown dwarf suggest that it has a mottled atmosphere with scattered clouds and mysterious dark spots reminiscent of Jupiter's Great Red Spot, as shown in this artist's concept. The nomadic object, called 2MASS J22081363+2921215, resembles a carved Halloween pumpkin, with light escaping from its hot interior. Brown dwarfs are more massive than planets but too small to sustain nuclear fusion, which powers stars.

Though only roughly 115 light-years away, the brown dwarf is too distant for any features to be photographed. Instead, researchers used the Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) at the W. M. Keck Observatory in Hawaii to study the colors and brightness variations of the brown dwarf's layer-cake cloud structure, as seen in near-infrared light. MOSFIRE also collected the spectral fingerprints of various chemical elements contained in the clouds and how they change with time.

ARTWORK: NASA, ESA, STScI, Leah Hustak (STScI)

The NASA Hubble Space Telescope is a project of international cooperation between NASA and ESA. AURA&rsquos Space Telescope Science Institute in Baltimore, Maryland, conducts Hubble science operations.


Contents

The revised Yerkes Atlas system (Johnson & Morgan 1953) [11] listed 11 G-type dwarf spectral standard stars however, not all of these have survived to this day as standards.

The "anchor points" of the MK spectral classification system among the G-type main-sequence dwarf stars, i.e. those standard stars that have remained unchanged over years, are beta CVn (G0V), the Sun (G2V), Kappa1 Ceti (G5V), 61 Ursae Majoris (G8V). [12] Other primary MK standard stars include HD 115043 (G1V) and 16 Cygni B (G3V). [13] The choices of G4 and G6 dwarf standards have changed slightly over the years among expert classifiers, but often-used examples include 70 Virginis (G4V) and 82 Eridani (G6V). There are not yet any generally agreed upon G7V and G9V standards.


Types of Stars

Lazarus star

A super nova remnant which, instead of being forced inward into neutron-star mode, survives as a normal star. After expansion into red giant phase, Lazarus stars collapse and undergo supernova for a second time.

Neutron Star

Usually type B-0 and measures only a few kilometres in diametre. An early main sequence star that has completed the nuclear burning processes often explodes. The reactive force of the explosion and the star's self-gravitation eject shell electrons (as in a white dwarf) and nuclear positrons. This leaves a neutroneum core, possibly covered by a thin degenerate matter shell.

Population I

Stars are old stars well down the main sequence (class F, G, K, and M stars) and short on heavier elements. Planetary systems accompanying Population I stars primarily consist of gas giants without accompanying satellites.

Population 2

Stars are younger stars showing traces of heavier elements, hydrogen, and helium. Planetary systems accompanying Population 2 stars include gas giants, stony worlds, satellite companions and planetoid and comet shells.

Red Giant Star

The red giant phase is common in the evolution of many less massive stars. When core hydrogen is exhausted, gravitational collapse ignites hydrogen shell burning outside the core. The star's envelope expands far beyond the photosphere limit. The star's atmosphere is relatively cool.

Runaway Star

A star with a velocity significantly different from its neighboring stars.

Supernova

When a massive young star exhausts its core hydrogen it undergoes second-stage gravitational collapse. The resulting core temperature increase leads to runaway nuclear burning of helium, carbon, nitrogen and an explosion that blasts the star's outer layer into space. Supernova explosions are the major source of metals and other galactic elements.

T Tauri Star

One manifestation of a star in formation undergoing initial nuclear burning.


Spectral Peaks and Valleys

If thermal radiation were the only source of light from a star, the star’s spectrum would be a nice smooth curve. However, actual spectra observed from stars have a series of peaks and valleys as shown in the spectrum below, meaning some of their light comes from “non-thermal” radiation – light emitted or absorbed by a process other than random jostling of atoms. In the next section, you will learn what this process is.

The spectrum below, from the SDSS spectral database, is a typical example of the spectrum of a star:

Many of these peaks and valleys have labels on them. You may recognize some of these labels as the symbols of chemical elements.

Every star has a unique pattern of peaks and valleys, and these patterns can be grouped into “spectral types” of stars. The traditional spectral types are denoted by the letters O,B,A,F,G,K,M (and some new spectral types have been added in the last couple of years…more on those later!)

Before you find out what these letters mean, take a shot at developing your own system for classifying stars based on their spectra.


Spectral Types

Using the technique of spectroscopy, stars can be classified by their colour (or temperature) into a series of letters which denote their spectral type. The hottest stars are denoted by the letter O, with the sequence progressing through B, A, F, G, K to the coolest M stars (see Figure 1). Properties and examples of each spectral type are listed below. Each spectral type is split further by the numbers 0 - 9 so that a B0 star is bluer (and therefore hotter) than a B9 star, which in turn, is slightly bluer than an A0 star.

Figure 1: A comparison of the spectra of different stellar classes, from left to right (400 to 700 nanometres 4000 to 7000 Angstroms). Thirteen normal stellar classifications are shown followed at the bottom by three more specialised classifications. Absorption lines can be seen as dark vertical bands.
Credit: NOAO/AURA/NSF
  • O-type stars have surface temperatures between 30,000 and 40,000 K. Using Wien's Law, we see that these stars have a peak wavelength of emission in the ultraviolet part of the electromagnetic spectrum. On average, 1 in every 3 million stars is an O-type star. The eastern (from Europe, the furthest left) star in Orion's Belt, Alnitak, is classified as O9.5 spectral type.
  • B-type stars have surface temperatures between 10,000 and 30,000 K. On average, around 1 in 800 stars are B-type stars. Rigel, the brightest and bluest star in Orion is of spectral type B8.
  • A-type stars have surface temperatures between 7,500 and 10,000 K. On average, around 1 in 160 stars are A-type stars. Vega, the brightest star in Lyra is of spectral type A0. When spectra are taken, it is A-type stars that display the strongest hydrogen lines, however this is more an indication of the star's temperature than of the abundance of hydrogen (which is generally 70 - 80 % of a star's total mass in all main sequence stars).
  • F-type stars have surface temperatures between 6,000 and 7,500 K. On average 1 in 30 stars are F-type stars. Procyon, the brightest star in Canis Major is of spectral type F5.
  • G-type stars have surface temperatures between 5,200 and 6,000 K. On average 1 in 12 stars are G-type stars. Our Sun is of spectral type G2.
  • K-type stars have surface temperatures between 3,700 and 5,200 K. On average 1 in 8 stars are K-type stars. Pollux, the lower of the two bright stars in Gemini is of spectral type K0.


Find out more about the technique of spectroscopy.

Read more about Wien's Law and how it relates to the electromagnetic spectrum .


Absorption Spectra From Stars

The light that moves outward through the sun is what astronomers call a continuous spectrum since the interior regions of the sun have high density. However, when the light reaches the low density region of the solar atmosphere called the chromosphere, some colors of light are absorbed. This occurs because the chromosphere is cool enough for electrons to be bound to nuclei there. Thus, the colors of light whose energy corresponds to the energy difference between permitted electron energy levels are absorbed (and later reemitted in random directions). Thus, when astronomers take spectra of the sun and other stars they see an absorption spectrum due to the absorption of the chromosphere.


Spectral Peaks and Valleys

If thermal radiation were the only source of light from a star, the star's spectrum would be a nice smooth curve. However, actual spectra observed from stars have a series of peaks and valleys as shown in the spectrum below, meaning some of their light comes from "non-thermal" radiation - light emitted or absorbed by a process other than random jostling of atoms. In the next section, you will learn what this process is.

The spectrum below, from the SDSS spectral database, is a typical example of the spectrum of a star:

Click on the image to see it full size

Many of these peaks and valleys have labels on them. You may recognize some of these labels as the symbols of chemical elements.

Every star has a unique pattern of peaks and valleys, and these patterns can be grouped into "spectral types" of stars. The traditional spectral types are denoted by the letters O,B,A,F,G,K,M (and some new spectral types have been added in the last couple of years. more on those later!)

Before you find out what these letters mean, take a shot at developing your own system for classifying stars based on their spectra.