Fisika Bintang

Bintang-bintang-Sebuah Pengantar

Bintang-bintang adalah serupa sebagaimana matahari kita, namun banyak aneka ragam variasi dari bintang-bintang ini. Satu hal menyangkut bintang  adalah, semua bintang lahir dari fusi nuklir yang terjadi di inti mereka.

Hampir setiap titik-titik cahaya yang berkelip di angkasa yang kita lihat merupakan bintang-bintang. Semua bintang tersebut berada di dalam Galaksi Bima Sakti kita ( Milky Way Galaxy). Sangat jarang bintang tersebut menyendiri berada di ruang antar Galaksi, dan normalnya bintang-bintang senantiasa berada dalam Galaksi ( galaxies).

Ada dua grup bintang-bintang:

• Bintang populasi II - bintang tua, kurang mengandung logam
• Bintang populasi I - bintang muda, bintang-bintang yang kaya dengan logam

Sedangkan ditinjau dari akhir hidupnya, bintang dikelompokkan pula menjadi dua, yaitu:

• Bintang normal ( Normal stars )- seperti matahari kita (Sun) - Bintang seperti ini akhir hidupnya menjadi Nebula Planet atau Bintang Katai Putih
• Bintang besar (Large stars) di mana akhir hidupnya adalah menjadi sebuah supernova atau berakhir menjadi Bintang Neutron dan Lubang Hitam (Black Hole)

Masa hidup bintang normal (normal star):

• Awan debu membentuk bintang deret utama yang akan menyala selama 10 milyar tahun
• Kehidupan bintang berakhir di deret utama untuk kemudian menjadi Raksasa Merah (seukuran orbit bumi) dan menyala selama 100 juta tahun
• Lapisan bakal bintang menjadi  Nebula Planet yang berlangsung selama 100.000 tahun
• Hanya inti bintang yang tetap sebagai bintang Katai Putih

Masa hidup bintang besar (large star):

• Awan debu membentuk sebuah bintang besar yang tetap bersinar di deret utama selama 50 juta tahun.
• Bintang tetap berada di masa hidup deret utama sebelum berakhir menjadi maha raksasa merah (kira-kira seukuran orbit planet Mars) dan bersinar dalam keadaan ini selama satu juta tahun.
• Core collapse can occur anytime after the million year Red Supergiant phase, and can go supernova
• All that is left is a supernova remnant (a wispy looking nebula) and a compact object - Neutron Star or Black Hole

Stars can be classified as living in groups as there are no "stray" stars existing in the Universe. There are actually three types of stellar populations:

• Open Clusters - a group of "new" stars in a group with a few hundred members
• Globular Clusters - a group of "old" stars in a group with a few thousand members
• Galaxies

Open clusters reside mostly within the disk of a galaxy while globular clusters exist outside the galaxy filling a space called the halo. This halo is actually part of the galaxy and it surrounds the entire galaxy.
Open clusters and globular clusters will be discussed in greater detail in their own sections.
Of course, a star does not have to be in an open or globular cluster but almost always a star will be a part of a galaxy.
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But how does the life of a star begin?

(Science Cartoons Plus)

The most abundant material in the Universe is Hydrogen. Clumped together in cold clouds, hydrogen atoms can join together to form molecular hydrogen. This only occurs at extremely low temperatures.

These molecular clouds are very difficult to detect because no emission occurs. Much of the interstellar reddening (where a star or galaxy appears more red) occurs because of these molecular clouds. The image on the left is Barnard 68, a dark nebula. This is what a molecular cloud "looks" like. Notice the stars are barely visible behind this cloud. However, it is possible to view what's behind the cloud using an infrared filter.

A cloud has to be "just right" before it can host a future star system. For successful gravitational collapse of a cloud to form a proto-star (a star that has not yet initiated fusion), two criteria must be met:

• Jeans Mass
• Jeans Length

These two criteria, discovered by James Jean in the 1940's, places restrictions on a collapsing cloud:

Jeans length basically states that a molecular cloud of a particular size can become unstable and begin collapse.

The Radius in the above equation is Jeans Length, the minimum radius of the cloud before self-gravitation occurs.

The table on the left give some examples of the density of molecular clouds and the resulting size of the cloud. Within a molecular cloud, the distribution of debris is not always even. Fragmentation is suspected to occur in clouds exceeding

100 Solar masses. Smaller clouds within the large cloud can form stars. These molecular cloud fragments also fall under the Jeans criteria, and does affect the overall molecular clouds ability to continue self-gravitation, but that is an advanced topic.
So what can cause a molecular cloud to collapse?

• Nearby stars that have ended their live in a supernova can send a shockwave stimulating collapse
• Density waves within a galaxy propagate through the spiral structures that can stimulate collapse
• Galaxy collisions can create huge gravitational forces to act of nearby clouds
• A nearby Wolf-Rayet star can stimulate collapse
• Sequential stellar formation - nearby stars forming close enough that their initial fusion can stimulate collapse

The cloud is collapsing, so now what?

As the molecular contracts under its own gravity, conservation of momentum forces the cloud to take on a disk shape, and it begins to spin. The very center of the cloud remains circular while the outlying gas forms a disk. Material from this disk is ejected perpendicular to to the disk as seen on the right.

(Image credit: Brooks/Cole Thomson Learning)

Once the proto-star reaches a certain temperature, the fusion of hydrogen atoms begins. The magic temperature is 10,000,000 Kelvin for our Sun, but for proto-stars of varying densities, the following formula applies:

It is important to know that there is a limit to stellar formation. The proto-star must fall within a:

• lower limit mass of 0.8 Solar masses
• an upper limit mass of 100 Solar Masses

A proto-star that is less than 0.8 Solar masses becomes a Brown Dwarf and a proto-star that exceeds 100 Solar masses becomes a Wolf-Rayet star - a very unstable star that cannot hold on to its outer layers.

As the Hydrogen atoms at the core of the proto-star are forced together by heat and pressure, the Coulomb Barrier is reached.

(Image credit: Brooks/Cole Thomson Learning)

The above image demonstrates that as the radius, r, or the space between protons is limited, a strong attractive force occurs.

(Image credit: Brooks/Cole Thomson Learning)

The Proton-Proton Chain begins with the fusing of hydrogen atoms into helium atoms - plus some gamma rays, neutrinos and photons.

Notice the times in the image above. It can take 1,000,000,000 years for the hydrogen atoms to exceed the Coulomb Barrier.

So what is the equation that demonstrates the energy produced by this reaction?

A T-Tauri star is a proto-star that has begun its fusion burning stage - with a bang and a shock wave that blows away any nearby debris close to the star.

I know this may seem like an overwhelming amount of information for an introduction, almost as complicated as trying to learn and understand the new system of POS software that came with the online store if there hadn't been some assistance available in learning the software along the way, but these pages are meant to aid you along as well. The following pages will revisit some of this material providing some insight with some real-world examples.

Stellar Astrophysics - the study of the stellar process - is not an easy subject, but hopefully this information will bring you one step closer to fully understanding the processes of not only our Sun, but all of the stars in the night sky.

Stars - Stellar Classifications

A star is classified by luminosity and color. luminosity is measure in magnitudes and color is measured by temperature. Edward Pickering and Willimina Fleming and a group of women (one of these women, Henrietta Leavitt, discovered the Cepheid variable star - important in distance measurements) cataloged thousands of stars according to spectra. While all the this number data was fine, it was two astronomers (working separately) that discovered a correlation to a stars spectra and brightness. Ejnar Hertzsprung and Henry Norris Russell created a plot of the stars and created what is now called the Hertzsprung-Russell diagram. We will look at this diagram later.

While our observations can determine luminosity and spectral classes, close observation and study of binary stars can also yield mass.


While stars are just point sources far from Earth, there is still much we can learn. For example, if the star is relatively close to us, we can determine its distance using stellar parallax:

By measuring the shift angle seen from Earth, we can determine distance. In case you don't know what parallax is, hold a pen or pencil arms length from your face and close one eye at a time while looking at something far away. The shifting of the pen is the parallax.

We can also determine the brightness of a star (as well as its luminosity) if we know its distance. We can use the Inverse Square Law:

However, notice that we need to know the luminosity of the star to determine brightness. We can determine the luminosity of the star using this formula:

but without knowing brightness, it seems we cannot determine luminosity. This is where our Sun comes in. We have the benefit of a frame of reference when comparing to other stars.

With the known values in place, its just a matter of some algebra.

Another classification for stars is magnitude - how bright does the star appear to us on Earth. There are two versions of magnitude:

• Apparent magnitude - brightness seen from Earth
• Absolute magnitude - how bright the star is if 10 parsecs from Earth

The magnitude scale is graded by numbers: 0 being bright, 6 being dim. Some values of comparison:

• Our Sun - -26.7 (that's a negative)
• Sirius - -1.4
• Naked eye limit - +6.0
• Binocular limit - +10.0
• Pluto - +15.1
• The Hubble Space Telescope limit - +30.0

The magnitude scale works out to be logarithmic, and the difference between each value is 2.512. That is magnitude 1 is 2.512 times brighter than magnitude 2 and magnitude 5 is 2.512 * 2.512 * 2.512 times dimmer that magnitude 2.

By knowing the distance and apparent magnitude to a star, we can learn the absolute magnitude. Also, if we happen to know the absolute magnitude, we can determine distance. The result of this is the distance modulus:

or:

By using filters (blue, green, red) and measuring the brightness of a star in the different filters, we can determine the color index of a star (and recall from the physics section that color and temperature go hand in hand):

If the B-V value is 1, the star is white. If the B-V index is less than 1, the star is more blue and if the B-V index is more than 1, the star is more red.

All of these tools led to a correlation: a stars size relates to how bright it and how hot it is.

It is this correlation that helped to create the Hertzsprung-Russell (or H-R) diagram:

(Image Credit: Pearson Education, Addison Wesley)

Going back to luminosity, there are six classes of luminosity:

• Ia - Bright Supergiant Stars (example: Deneb)
• Ib - Supergiants (example: Antares)
• II - Bright Giants (example: Canopus)
• III - Giants (example: Capella)
• IV - Subgiants (example: Beta Cru)
• V - Main Sequence (example: Vega)

Classes can be categorized like: Ia, Ib, Ic, IIa, IIb, IIc, and so on (sub-a being brighter that sub-b).
In addition, notice the x-axis of the H-R diagram. This is the spectral class - the result of the Harvard team.

• O - O type stars are the brightest and the live the shortest
• B - B type stars are blue white and also burn bright, but not as bright as the O type
• A - A type stars are less bright, a little larger than our Sun, but still burn hotter
• F - Brighter than our Sun and a little hotter
• G - Our Sun is a G type star
• K -  dimmer that our Sun, will burn longer because temperature is lower
• M - the dimmest stars, will burn for a long time

(Image Credit: Pearson Education, Addison Wesley)

The famous pneumonic helps astronomers remember this sequence: Oh Be A Fine Girl (or Guy) and Kiss Me. Who says scientists don't have a sense of humor?
One final interesting formula: a quick and dirty correlation between a stars luminosity and magnitude is:

All that is left is to determine the mass of a star. We now know that bright stars are very large and dim stars are small, but without an accurate measure of mass, we can only speculate. This is where binary stars come into play.
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A binary star is a multiple star system bound by mutual gravitation. Both stars will revolve about a common point. By using Keplerian math, we can determine the mass by studying this movement. If we know the spectral class of the stars in question, we can assign standardized values (like the H-R diagram) that will give is the mass of all stars.
There are four main classes of binary stars:

• Visual binary stars - stars that we see on Earth as being binary
• Spectroscopic binaries - binary stars that are only seen using spectroscopy
• Eclipsing binaries - binary stars at our line of sight
• Accreting binaries - close pairs that "feed" off each other

A visual binary star system can either be a real binary, or one that looks that way because of our view on Earth. A true binary will move something like this:

(Image credit: Brooks/Cole Thomson Learning)

In order to find the mass of a visual binary:

(Image credit: Brooks/Cole Thomson Learning)

m in this case is mass and P is the period of the orbit.
A spectroscopic binary star is one that is not seen visually, but long term observation records a shift in the stars spectrum:

(Image credit: Brooks/Cole Thomson Learning)

Sometimes if the orbit of the binary star is facing us, we may not be able to see spectroscopic changes, but we will see brightness changes. Long term observation will reveal periodic changes in overall brightness:

(Image credit: Brooks/Cole Thomson Learning)

This same technique is used for the detection of exoplanets.

The final class of binary star is the accreting binary. If the pair of stars are close enough, the atmosphere of one star can pour onto another. The point of no return in this case is the outer Lagrange point (not seen in the image). If material breaches the outer Lagrange point and contacts the inner Lagrange point, material will begin to flow onto another (that is what I mean by the point of no return).

(Image credit: Brooks/Cole Thomson Learning)

A common companion star to an accreting binary system is a neutron star or a white dwarf - both covered in stellar evolution.

Some final bits: binary star system are actually quite common - Sir William Herschel discovered 10,000 binary star systems. In addition, binary stars are really part of multi-star systems. Many stars have more than one companion. A triple star system is also common.

To sum up, here is a nice chart I found that demonstrates the process of stellar classification:


Stars - Stellar Populations

Stars can be grouped by various populations. The most obvious being star cluster: globular clusters and open clusters.

More specifically stars can be divided by main population based on Metallicity.

• Population I stars - new stars that contain numerous heavy metals in their atmosphere
• Population II stars - old stars that contain little heavy metals in their atmosphere

When astronomers think about metals, they are not referring to iron and nickel (while they certainly are metals). To an astronomer, any element heavier than helium is considered a metal. The main reason for this is that the only elements that existed in the early Universe was hydrogen and helium. Other "heavier" elements were formed in the process of stellar evolution.
Population II stars were believed to have formed first. These stars occupy the globular clusters that reside in the halo of the galaxy. However, it should be noted that the search is on for Population III stars. Astronomers suggest that the very first stars to ever form in the universe were Population III - that only burned hydrogen and helium only. The suggested comes from the determination that most Population II stars do have some heavy elements.
Some characteristics between a Population II star versus a Population I star is:

• Population II stars burn hotter
• Population II stars burn faster

Astronomers believe this has to do with the opacity of the stellar atmosphere. More metals mean a more opaque atmosphere in a Population I means less energy escapes (when compared to Population II stars anyway).
So how does a Population I star contain metal when a Population II star does not?
We will cover this in stellar evolution, but much of the heavy elements in our Universe today is created when a giant star end its life in a supernova. The intense heat for this creates in the upper atmosphere of stars much of the elements we know - like iron, gold, even fluoride (yes, the same stuff in your toothpaste).
These elements disperse themselves to nearby molecular clouds. When that cloud undergoes contraction and give birth to a new star, the end result is a Population I star - one that is now metal rich.
One consequence of a metal rich star is that they are likely to contain a system of planets!