Visions of the Cosmos

Astrophysics

STARS

Types of Star


The way in which a star behaves largely depends upon its mass.

Gas Giant Planets

Although Gas Giant Planets are not stars their overall composition is similar to stars like the Sun in that they are largely composed of hydrogen and helium with much smaller amounts of other elements.  The only two true examples in our Solar System are Jupiter and Saturn.  Many Planets ranging from Saturn Masses to Jupiter Masses and well above have been detected orbiting other stars.  Gas Giant Planets do not reach high enough masses for temperatures to initiate thermonuclear reactions.

Brown Dwarfs (sometimes spelt Brown Dwarves)

For some time, astrophysicists had predicted the existence of objects which have masses in between Jupiter sized bodies and very low mass stars    The first brown dwarf was detected in 1995.

The lower mass limit of a brown dwarf (and the notional boundary between these objects and a planet) is defined as 20 Jupiter masses (20 Jm).

High mass brown dwarfs can support fusion for about ten million years. It is not hydrogen that is burnt but heavy hydrogen (deuterium).  Lower mass brown dwarfs  reach  temperatures between 500,000 and 3,500,000 degrees Celsius in the core and are only able to burn deuterium. 

Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which achieve the high temperature necessary for fusing hydrogen, rapidly deplete their lithium.  The temperature necessary for this reaction is below the temperature necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in the whole volume of the star is depleted. Therefore, the presence of the lithium line in a candidate brown dwarf's spectrum is a strong indicator that it is indeed a brown dwarf rather than a low mass red dwarf star.

The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the lithium test.  However the test is not infallible since the most massive brown dwarfs are hot enough to burn lithium as well as deuterium when they are young although they are not hot enough to burn hydrogen.  Dwarfs of mass greater than 65 MJ can burn off their lithium by the time they are half a billion years old thus this test is not perfect.

However, lithium is also seen in very young stars, which have not yet had a chance to burn it off. Heavier stars like our Sun can retain some lithium in their outer atmospheres, which never get hot enough for lithium depletion, but those are obviously distinguishable from brown dwarfs by their size.

.Brown dwarfs can have a surface temperature of between 1500 K and 1000 K. Methane and water molecules can exist within this range.

Red Dwarf Stars

These are the first true stars in that the temperatures in the core can reach up to 15,000,000 degrees which is more or less the temperature at which hydrogen can burn to helium.  These stars are known as red dwarf stars and are also called stars of spectral class M.  They range in mass between 0.08 and 0.7 solar masses.. They take a much longer time than Sun-like stars to reach the fusion stage to condense from a mass of dust and gas. They spend millions of years condensing (the smaller the mass the longer it takes).

Once their cores are hot and dense enough fusion starts and proceeds at a leisurely pace. Fusion may continue for as long as the present lifetime of the universe. Their indefinite lives mean that red dwarfs are now relatively plentiful in old globular clusters and in our galaxy. Even if the Universe lasted long enough red dwarfs could not become red giants. They do not have the mass to create the conditions in the core to allow the fusion of helium to carbon.   Thus means they would never reach the same white dwarf stage as our Sun but would remain at a helium stage forever.

Red dwarfs have surface temperatures of around 3500 K. The lower mass limit for a red dwarf is, by definition, 0.08 solar mass as that was thought to be the minimum mass that is required to produce the core temperature and pressure that will support the nuclear fusion of hydrogen.

Solar Mass Stars

Stars of between 0.7 and 1.2 solar masses have similar spectra. They remain on the main sequence, fusing hydrogen in a stable manner for eight to fifteen billion years. These stars, such as our Sun, when they run out of hydrogen, are massive enough to burn helium to carbon and to support some burning between carbon and helium to give oxygen.  They become red giants and cast off 'planetary nebula'. The remnant hot core will slowly cool as a white dwarf. mainly consisting of carbon and oxygen.

Massive Stars

Although the Sun is in the top 15 %  of the most massive stars there are quite a number of stars somewhat more massive.  There are however a small number of stars far more massive than the Sun.  They burn far faster than the Sun and end their comparatively short lives in the production of massive white dwarfs, or in the most extreme cases, stars of over 8 solar masses undergo enormous supernova explosions which end in the production of neutron stars or in the most extreme cases of all black holes.    These very massive stars will be discussed in detail in the next section of the web-site dealing with thermonuclear reactions.

 

Hertzsprung - Russell Diagram

In the early 1900's, Ejnar Herstzprung and Henry Norris Russell independently made the discovery that the luminosity of a star is related to its surface temperature. (They actually used a quantity known as the Magnitude of the star and the Spectral Class of the star.) A schematic Hertzsprung-Russell diagram is shown below. Courtesy NASA

Hertzsprung and Russell both used the spectral class (which is related to the temperature) in their plots. They ordered the stars as O, B, A, F, G, K, and M. Since the O stars are the hottest, this means that in the HR diagram, the temperature axis is unusual in that the temperature decreases as one moves to the right.

The Hertzsprung-Russell diagram. The diagram shows the main sequence, red giants, supergiants, and white dwarfs. In addition, we identify the location of the Sun, the twelve brightest northern hemisphere stars and the white dwarf companions of Sirius and Procyon.

The diagram Courtesy NASA

 

Very many thanks are due to Dr Bob Gomersall of the BTL Group. in Shipley, West Yorkshire for his kind permission to reproduce the beautiful diagram on the left. Acknowledgement for illustration to I R and W D Gomersall

Web-site http://www.telescope.org/btl/lc4.html

Click illustration for enlargement

Note the main sequence stars all lie on a clear curve.  They are referred to perhaps rather inaccurately as dwarfs.  Epsilon Eridani is an orange dwarf,  the Sun is a yellow dwarf, Sirius is a white dwarf.  What can not be indicated in this diagram is the fact that the majority of stars are red dwarfs.  The Sun and other orange, yellow and white stars make up most of the rest with only a small number  in the blue.  Green stars have been reported but are very rare.  The white dwarfs are extremely hot but very small.  The red giants are large stars like the Sun at the end of their live before they collapse into the white dwarf stage.  The Red and Blue Supergiants are massive stars which are close to the end of their life span and finally explode as supernovae and yield neutron stars and a cloud of material containing large amounts of higher elements which are added to the interstellar medium and form part of the material of future stars and planets.  A few very massive stars collapse into black holes and explode as hypernovae.

Table of Typical Well Studied Stars

 

Star Constellation Spectral Type Mass in Mo Diameter (Sun =1) Surface  Temperature Luminosity Distance Age of Star years Type
Zeta Puppis Puppis O51a 59 20 47,400 790,000 1,400 4.0 X106  Blue Super Giant
Rigel Orion B8 17 70 11,000 66,000 800   blue
Vega Lyrae A0V  2.6 2.73 9,300 51 25.3 3.5 X108 blue white
Sirius Canis Major A1V  2.02 1.71 9,900 25.4  8.6 2.3 X108 white
Procyon Canis Minor F51V-VDA  1.5 1.86 6,650 7.73  11.4 1.7X109 orange white
Sun   G2V  1.0 1.00 (1.390,000km) 5,785 1.00   4.5X109 orange
Tau Ceti Ceti G8V 0.81 0.83 5,344 0.59 11.89 1.0X1010 orange yellow
Epsilon Eridani Eridanis K2V 0.85 0.84 5,100 0.28 10.5 5.0 X108 orange star
Proxima Centauri Centaurus M5.5V 0.12 0.15 2,670 5-12 Χ 10-5 4.22 1.0X109 red dwarf
High Mass Brown Dwarf     60 X Jupiter about same as Jupiter         red brown
Low Mass Brown Dwarf     20 X Jupiter about same as Jupiter         magenta
Jupiter     0.001 0.102 (142,600km)          
 

As the table above shows there is very little difference in the diameters of brown dwarfs and gas giant planets.  The diagram on the left shows an artist's rendition comparing stars, brown dwarfs, and planets to the same size scale. From left to right is the limb of the Sun, a very low mass star (red dwarf), a pair of brown dwarfs, and the planet Jupiter. These objects have masses ranging from 1000 times that of Jupiter (for the Sun) through 75, 65, 30, and 1 Jupiter mass, respectively. Despite the range in mass, all four of the low-mass objects are approximately the same size, ten times smaller than the diameter of the Sun.                     Diagram Courtesy UCLA/NASA/JPL

Within 26 light years from the Sun and visible from the northern hemisphere there are 4 blue stars, 1 green-tinted star, 5 yellow stars (including the Sun), 22 orange stars, 87 red stars, and 9 white dwarfs. There are in addition many brown dwarfs.   The brown dwarfs are composed of a few red and redder dwarfs along with lots of magenta dwarfs (or cooler objects). Despite the fact that there are at least as many brown dwarfs as stars, the stars are responsible for most of the mass off objects in the volume studied.

The mass of the Sun is approximately 1.98 X 1038  kilograms.  It is used as a unit of mass in astronomy Mo.  Jupiter is only 0.001M and the Sun is 300,000 times more massive than the Earth

Morgan Keenan Spectral Types

This stellar classification most commonly used. is the Morgan Keenan.  It is also known as the Yerkes classification. The common classes are normally listed from hottest to coldest (with mass, radius and luminosity compared to the Sun) and are given in the following table.  The colors in this table are greatly exaggerated for illustration. The actual colors of the listed stars are mostly white with a faint tint of the color indicated; stars' colors are often too subtle to notice, particularly when they are near the horizon.
Class Temperature Star colour Mass Radius Luminosity Hydrogen lines
O 30,000 – 60,000 K Bluish ("blue") 60 15 1,400,000 Weak
B 10,000 – 30,000 K Bluish-white ("blue-white") 18 7 20,000 Medium
A 7,500 – 10,000 K White with bluish tinge ("white") 3.1 2.1 80 Strong
F 6,000 – 7,500 K White ("yellow-white") 1.7 1.3 6 Medium
G 5,000 – 6,000 K Light yellow ("yellow") 1.1 1.1 1.2 Weak
K 3,500 – 5,000 K Light orange ("orange") 0.8 0.9 0.4 Very weak
M 2,000 – 3,500 K Reddish orange ("red") 0.3 0.4 0.04 Very weak

Acknowledgement for table and illustration of Morgan Keenan Classification of stars  Wikipedia

Duration of the Life of a Star

The more massive a star is the faster it burns and the shorter is it's life span in the main sequence.

Metallicity

  There are two ways of stating the composition of the Sun:- 

Percentage by number of atoms 94% hydrogen, 6% helium 0.11% of oxygen, carbon and nitrogen with 0.02% other elements.

Percentage by mass 78.5% hydrogen, helium 19.7%, oxygen 0.86%, carbon 0.4%, nitrogen, iron and other elements 0.54%

Astronomers use the word METAL in a totally different way to chemists.  The word metal in astronomy means ALL elements except hydrogen and helium.  This of course includes elements such as carbon, nitrogen, oxygen, chlorine, sulphur, phosphorus and all the other elements that are classified as non-metals in the chemical, physical, geological and biological sciences.  Although to call these elements METALS seems quite absurd to the science of chemistry it has become so much part of the vocabulary of astronomy that it is unlikely that a better term will ever be found.   Even in astronomy the term is not used for terrestrial planetology and is really only applicable to STARS.

The term METALLICITY is used in describing the amount of METALS (in the astronomical sense) a star contains. It is very difficult to measure the composition of a star - even our own Sun .  the photospheric composition of the Sun given in the current Wikepedia is given below.  It does not agree closely with the figures given in above which were taken from Nick Strobel'sAstronomy  Notes web-site  http://www.astronomynotes.com/copyright.htm.  Composiition figures must of necessity be estimates only.   The photospheric composition of the Sun by mass is approximately Hydrogen 73.46%, Helium 24.85%, Oxygen 0.77%, Carbon 0.29%, Iron 0.16%, Sulphur 0.12%, Neon 0.12%, Nitrogen 00.09%,Silicon 0.07%, Magnesium 0.05%

The metallicity of a star is hard to estimate and varies enormously.  Compared to the Sun Sirius is estimated at 190%, Procyon 110%, Vega 63% and Epsilon Eridani 0.49-0.60.

The importance of metallicity comes into play when considering the possibility of large terrestrial type planets like our Earth.  As a generalisation the higher the metallicity the greater the likelihood of Earth-like planets.

Binary and Multiple Star Systems

Our Star the Sun is a single star.  However there are many star systems which consist of two or more stars orbiting a common centre of gravity.

An example of a binary or perhaps a triple star system is Alpha Centauri.   It consist of two Sun-like stars - one Alpha Centauri A is a yellow white slightly more massive than the Sun and is of the same spectral type(G2).  Alpha Centauri B is a deep yellow (K1 spectral type) and is slightly less massive than the Sun.  The two stars orbit each other and take 80 years to complete an orbit.  The two stars are 4.39 light years away from the Sun.  A third star known as Proxima Centauri is nearer to the Solar System by 0.17 light years so it is very far away from the two larger stars.  It is a very faint M5 red dwarf only 0.12 times the mass of the Sun.

Binary and multiple star systems come in all sorts of varieties.  In the case of Alpha Centauri the two main stars are both Sun-like whilst the third star in the system is a very small red dwarf.  In the case of Sirius there is one star which is two and a half times the mass of the Sun called the Dog Star.  It is accompanied by a white dwarf Sirius B sometimes called the Pup.  Sirius B is about 42,000 kilometers in diameter and has a density 125,000 times that of water.  White dwarf stars have already been discussed in the web-page on the Sun and the eventual fate of the Sun after it completes the main sequence stage.

Another example of a binary was given in the web-site on the page on Extra-solar Planets.   55 Cancri A very similar to the Sun.  So far away is the red dwarf companion that we can almost think of Cancri A as being a single star capable of accommodating a planetary system.

It used to be believed that binary systems where the stars were close together would be unlikely to have planets.  However, recently a large planet was detected in a triple star system     It was discovered by Maciej Konacki of the California Institute of Technology in Pasadena. After surveying only about 20 multiple stars with the 10-metre Keck I telescope in Hawaii, he found signs of a planet in a triple star system called HD 188753. The system lies about 149 light years away in the constellation Cygnus.

The new planet is at least 14% more massive than Jupiter and orbits very close to a Sun-like star, while two further lower mass stars circle farther out, at roughly the distance between Saturn and the Sun.  The two other stars would rise together as smaller discs glowing orange and red. "The environment in which this planet exists is quite spectacular," says Konacki. "With three suns, the sky view must be - literally and figuratively - out of this world.!"

According to Konacki, this planet must witness dramatic multiple sunrises. In its skies, the main star would look yellow-white like the Sun, but loom much larger. “The star would look 25 times larger than our Sun because the planet orbits very close to the star”  Konacki told New Scientist. as reported to Hazel Muir and published in the New Scientist in 13 July 2005 issue.

In the illustration on the left the artist's impression shows a scene from one of the gas giant planet's moons.  Watch a multiple sunset, viewed from the hypothetical moon around the new planet. Image artist's impression Courtesy NASA/JPL/Caltech  Click illustration for enlargement

 

The next section of the web-site 'The Magic Furnace' will deal with the thermonuclear production of the chemical elements in the cores of very massive stars. It is in the process of preparation

Stars

The Solar Wind

   Superstars and Supernovae

   The  Magic Furnace