WHERE DO STARS FORM?

Stars form in the densest regions of the interstellar medium, or ISM, called molecular clouds. The ISM is the name given to the gas and dust that exists between the stars within a galaxy. It is 99% gas and 1% dust, by mass.

Properties of ISM gas in our region of the Milky Way Galaxy:

70% hydrogen 28% helium 2% elements heavier than helium

Molecular clouds are perfect star-forming regions because the combination of these atoms into molecules is much more likely in very dense regions.

This photograph shows the Orion Nebula, an interstellar cloud in which star systems - and possibly planets - are forming. Our own solar system presumably formed as gravity caused the collapse of a similar large cloud of gas. The piece of cloud that formed our Solar System is known as the solar nebula.

WHY DO STARS FORM?

A star forms when a molecular cloud collapses under its own gravity forming a dense core sustained by nuclear fusion. This happens only when the force of gravity pulling in exceeds the outward push of pressure. High-density molecular clouds have stronger forces of gravity pushing in, making it easier to overcome the total pressure within the cloud.

Once started, the collapse of the solar nebula continues because the force of gravity exerted on the cloud grows stronger as the cloud shrinks in size. The universal law of gravitation is written mathematically:

The force of gravity between two objects: F=GMM/d^2

Fg is the force of gravitational attraction, M1 and M2 are the masses of the two objects, and d is the distance between their centers. The symbol G is known as the gravitational constant. Its value is G = 6.67 · 10-11kg·m2/s2.

Before its collapse began, the gas that made up the solar nebula was probably spread out over a roughly spherical region a few light-years in diameter. This gas was extremely low in density and extremely cold.

Possible causes for the start of a collapse:

Shock wave from a nearby exploding star Collision of 2 molecular clouds

GRAVITY VERSUS PRESSURE

In order for a cloud to collapse, the strength of gravity pulling in must exceed the strength of pressure pushing out. Let’s see how these forces battle each other mathematically.

Thermal pressure is directly proportional to both the temperature T and number density n within a cloud.

P = nkT is known as the ideal gas law. P represents gas pressure, number density n is equal to the number of particles contained within each cubic centimeter, k is Boltzmann’s constant and T represents temperature measured in Kelvin. Let’s start by finding out more about the number density. If each gas particle has a mass m, then M m gives the total number of particles within a cloud of mass M. The

number density is then found by dividing this number by the volume of the cloud. For simplicity, we will use a spherical cloud. The volume of a spherical cloud of mass M and radius r is

The force on a spherical cloud due to pressure is equal to the thermal pressure times the area of the cloud.

This is the expression we will use as the force on the cloud from thermal pressure. Now, in order to find an equation for the force due to gravity we have to think about the cloud in a slightly different manner. Imagine slicing the same spherical cloud into halves, so that each half exerts a gravitational force on the other half. The mass of each half is M/2, and the separation between their centers is approximately equal to r.

Newton’s law of gravity says that the force due to gravity is . With the mass of each half set to M/2, the force due to gravity is:

Where G is the gravitational constant.

These equations for the force due to pressure and the force due to gravity show how each depend differently on the mass, temperature and density of the cloud. An increase in the mass of the cloud increases both the gravity and the pressure. However, the force of gravity increases more since it is proportional to the mass squared. The mass at which the forces of gravity and pressure are equal is called Jean’s mass, after the British physicist Sir James Jeans.

Where temperature T is measured in Kelvin and number density n is stated in units of particles per cubic centimeter. In a cloud with mass M greater than Jean’s mass, gravity is stronger than thermal pressure and the cloud will collapse. The opposite is true for a cloud with mass M less than Jean’s mass. This is the method scientists used to find the minimum mass of newborn stars.

As the solar nebula shrinks in size, its density, temperature and shape all undergo dramatic changes.

HEATING: The temperature of the solar nebula increases as it collapses. As the cloud shrinks, its gravitational potential energy is converted to the kinetic energy of individual gas particles falling inward. These particles crash into one another, converting their kinetic energy into thermal energy.

SPINNING: Like an ice skater pulling in her arms as she spins, the solar nebula rotates faster and faster as it shrinks in radius. This increase in rotation rate represents conservation of angular momentum. The rotation of the cloud may have been imperceptibly slow before its collapse began, but the cloud's shrinkage makes fast rotation inevitable.

FLATTENING: The solar nebula has flattened into a disk. This flattening is a natural consequence of collisions between particles in a spinning cloud. A cloud may start with any size or shape, and different clumps of gas within the cloud may be moving in random directions at random

speeds. When the cloud collapses, these different clumps collide and merge, resulting in a flattened rotating disk.

WHAT SLOWS THE CLOUD COLLAPSE?

As the collapse continues, density increases and the molecules inside begin to interact more and more. This extra interaction causes the temperature to increase. Since pressure depends on density and temperature, the pressure increases.

Once the pressure gets high enough, it begins to rival the strength of gravity and the collapse of the cloud slows down. Eventually, the cloud of gas becomes a protostar: an infant star that has not yet begun to fuse hydrogen in its core.

NUCLEAR FUSION

A protostar is not considered a star until it gets its energy from nuclear fusion instead of gravitational contraction. Gravitational contraction is the process of a cloud's gravity overcoming its internal pressure and causing a collapse. The core temperature of an object must reach a minimum temperature before the atoms inside will fuse.

Minimum temperature for:

Hydrogen to fuse into helium: 10 million °Kelvin 3 helium nuclei to fuse into carbon: 100 million °Kelvin

The rate of fusion increases until the amount of energy produced in the core equals the amount of energy radiated from the surface. The perfect balance between energy produced and energy released stops the protostar from collapsing further.

MASSES OF NEWBORN STARS

The number of low-mass stars is much higher than the number of high-mass stars.

Minimum mass of newborn star: 0.08 times the mass of the Sun Maximum mass of newborn star: 150 times the mass of the Sun

The diagram shows the relative number of stars for every high-mass star. For every star 10-150 times the mass of our Sun, there are 50 stars close to the mass of our Sun. Our Sun is pretty average in size and mass.

Stages of Star Formation

Stage 1 Cloud collapse and fragmentation clouds are non-uniform densest pockets collapse first, leading to fragmentation

stars form in groupsStage 2

Continuing collapse of each fragment gravitational collapse should increase Kinetic energy of cloud

(motion=heat) added heat goes into "exciting" atoms-'radiative cooling'

Stage 3 Fragmentation ceases, a protostar is born central regions of the fragment become opaque to their own

radiation trapped radiation=poorer cooling central temp is greater than surrounding gas dense opaque inner region=protostar

Stage 4 Protostar I protostar continues to contract and gain mass temp increases pressure increases which leads to slowing in collapse release of gravitational energy=high luminosity equatorial plane (disc) formed (planets could form)

Stage 5 Protostar II contraction proceeds slowly temperature and pressure increase

Stage 6 Star is born Helium production in core nuclear reactions radius and temp are greater than sun's contracts slowly as settles into equilibrium

Stage 7 Main sequence star reaches equilibrium 30million+ years to reach main sequence central T=15million degrees surface T=6000 degrees

Black Hole

Mass (in solar

masses)

Diameter of Black Hole (in miles)

Diameter of Black Hole

(in kilometers)

Comment

3 11 18 smallest stellar mass black hole10 37 60 medium stellar mass black hole100 370 600 largest stellar mass black hole

1,000 3,700 6,000intermediate mass galactic black

hole

1 million 3.7 million 6 millionblack hole at galactic center of the

Milky Way Galaxy1 billion 3.7 billion 6 billion black hole in a supermassive quasar

Black Hole Fun Facts1. The idea of a "dark star" from which no light could escape was first introduced in 1783 by John Michell from England, and Pierre-Simon Laplace from France.2. The term "Black Hole" was first used in 1967 by John Archibald Wheeler. He also coined the term "Worm Hole."3. The closest black hole to Earth is Cygnus X-1, which is 8,000 light years away.4. Nothing can escape a black hole, but they do release x-ray jets at their poles. Over time, they shrink and will eventually disappear, or "evaporate."5. The "escape velocity" or speed needed to escape a black hole's gravity, is faster than the speed of light. But...matter cannot travel that fast! So...Speed of Light = 186,000 miles/second

What is a Black Hole?* It is a region where matter collapses to infinite density and where the resulting curvature of space-time is extreme. It is a region in space from which nothing can escape, except for the radiation that leaks out.* There are two basic kinds of black holes: Stellar and Supermassive

What is a White Hole?*A white hole is the reverse version of a black hole. In fact, if a black hole can only suck things in, a white hole can only spit things out.* Einstein's equations for black holes still function in reverse. But, white holes likely do not exist, because producing a white hole is just as difficult as destroying a black hole.

What is a Wormhole?* Wormholes have not been proven to exist. But, the possible combination of a black hole linked to a white hole is called a wormhole.* In effect, if the interior of an electrically charged or rotating black hole could join up with a white hole, then you would fall into the black hole and pop out of the white hole.* The white hole may be very far from the black hole; it may even be in another universe!* They sound pretty cool, but even if a wormhole formed, it would not be stable. Even the slightest disturbance would cause it to collapse.* If wormholes exist and are stable, they would be quite unpleasant to travel through. The X-ray and gamma radiation that pours into the wormhole would cause you to get fried. Toast. Crispy. Ouch!

What is a Tachyon?* The Latin word "tachys" means fast. A tachyon is a particle that travels faster than the speed of light. Tachyons have never been found to exist, although scientists are searching for them.* Albert Einstein's theory of relativity says that nothing traveling slower than the speed of light can ever travel faster than 186,000 miles per second (the speed of light). But FTL's (faster than light) could potentially exist IF they never traveled slower than the speed of light.* Luxons travel at the speed of light. The name given for the massless particle that travels at the speed of light is the "photon." Gravitons and possible neutrinos are other kinds of luxons.* The Latin word "tardys" means slow. Tardyons are things that travel slower than the speed of light. They include things like protons, electrons, neutrons and other subatomic particles of matter.

* If an alien, made entirely of tachyons, came toward you from his spaceship, you would see him arrive before you saw him leave his ship. The image of him leaving his ship would take longer to reach you than his actual FTL body. The creature would also visually appear to be traveling away from you. This means you would see a tachyonic alien as receding backward in time!

Planetary Nebula 95% of all stars that we see in our own galaxy, the Milky Way,

will ultimately become "planetary nebulae." This includes the Sun.

Planetary nebulae are formed when a red giant star ejects its outer layers as clouds of luminescent gas, revealing the dense, hot, and tiny white dwarf star at its core.

The other 5% of stars -- that is, those born with masses more than eight times (8x) larger than our Sun -- end their lives as explosive supernovae.

FYI: The name "planetary nebula" is a misnomer (not a good name choice)...

The name "planetary nebula" arose long ago when Sir William Herschel looked through his small, poor-quality telescope and saw these objects as compact, round, green-colored objects that reminded him of the view of his newly discovered planet, Uranus.

Planetary nebulae are not made of planets, and no planets are visible within them.

Rather, they are the gaseous and dusty material expelled by a small to medium-sized dying star.

A far better name for these objects would be "emission nebulae" because they emit gases in a small burst, sort of like a "burp" of gases suddenly thrown outward into space.

The Eagle Nebula Horsehead Nebula

Rotten Egg Nebula Cat's Eye Nebula