Gravity causes objects to move towards one another, in the absence of other forces. Gravity is much weaker than the other forces. In outer space, it is usually the only one of the four that can act over scales larger than the solar system. It is a long-range force and it always causes attraction between objects. The heavier the masses involved, or the closer together they are, the stronger the effect of the gravitational force and the higher the velocities achieved by the infalling particles.

Stellar Formation

As enormous clouds of cool gas collapse under the force of gravity, matter is compressed into swirling clouds that force particles to collide at a faster and faster rate, generating heat. At the core of these massive swirling clouds of hot gas, the force of gravity eventually generates enough energy that atomic nuclei are moving fast enough so that they can fuse together. Getting from cool gas clouds to a star with nuclei fusing together is a very complicated process that involves several steps. The collapsing gas will always heat up and tends to balance the force of gravity, halting the collapse. In order to continue the collapse, various cooling processes become important at different temperatures so eventually a star does form.

When the nuclei of atoms smash together or break apart, they often change their mass in the process. This gain or loss of mass corresponds to a loss or gain of energy, as well. Einstein discovered that this relationship follows the equation, ΔE=Δmc², where Δm is the change in mass of a particle or its constituents, c is the constant speed of light and ΔE is the equivalent energy. In summary, gravity is the force that creates the pressure to fuse atoms, which makes the stars shine.

A protostar is a star in the first stages of development. Protostars are not detectable in visible light and mostly emit infrared radiation. As gravity compresses the core of a protostar, the temperature goes higher and higher. Eventually the temperature is high enough that the star starts fusing hydrogen into helium. When the outward pressure produced by the heating of the gas by fusion energy balances gravity, a stable star is formed. The continued fusion of the stable star creates enough energy for the star to shine in visible light.

Stellar Classification

Known stars have a range of masses, from roughly 0.1 solar masses to somewhat over 100 solar masses. The higher the mass, the larger the gravitational force exerted on the atoms in the star and the larger the pressure that can be created. The central temperature of a star increases as mass increases. The surface temperature, which determines the color of a star, is much lower, but is related to the central temperature. The temperature of the surface of stars is measured in degrees Kelvin (K), that is, on an absolute temperature scale. The lowest mass stars have surface temperatures as low as 3,000 K and emit red light; the surface of the Sun is around 5,600 K, emitting yellow light; and the surface temperatures of the most massive stars exceed 30,000 K, emitting mostly blue light.

Spectral Classes of Stars: Stars are classified by color, which relates to their surface temperature. The color of a star can be used to determine its radius. Red stars are the smallest and coldest stars, and blue stars are the largest and hottest.

Lifespan of Stars

The lifespan of a star is determined by how long it can fuse hydrogen in its core. Small red stars burn at a very slow rate and can last for tens of billions of years. Yellow stars like the Sun are believed to last for ten billion years. Large blue stars fuse hydrogen at a much faster rate, and last only around ten million years. When the cores of small red stars run out of fuel, they slowly dim, fading into what are called red dwarfs. When stars the size of the Sun and larger run out of hydrogen, the outward pressure of fusion is no longer produced, and gravity causes the stellar cores to contract. Gas in the outer part of the star collapses towards the core and heats up: now fusion can start again in this unburned outer part of the star. The fusion pressure now exceeds the gravitational force in the outer part of the star and the star expands. The surface of the star cools and the star turns red. Stars like the Sun thus expand before they die. The red giant phase of solar type stars and the red super giant phase of more massive stars last much shorter amounts of time than the stable stars from which they came. It is often said that low-mass stars are cool, red, small in radius, and long-lived; high-mass stars are hot, blue, large and last a very short time.

Planetary Nebulae

Red giant stars are very unstable. As the cores run out of fuel and fusion stops, the outer envelopes collapse onto the core. The heating of the envelopes eventually causes a new region of fusion of hydrogen to helium and the heat generated causes the low-density envelope to expand. The Sun, for instance, will eventually expand in its red giant phase until the edge reaches the orbit of the outer planets. When the hydrogen burning ceases in the outer envelope, the material surrounding the core will drift off, forming a glowing cloud of gas known as a planetary nebula. The stellar core, having no fusion underway, contracts under the force of gravity and becomes very hot. The core of the Sun will be left behind as a cooling white dwarf star.

Formation of the Elements and Supernovae

For stars that are at least eight times more massive than the Sun, the process is quite different. Fusion stops when the hydrogen is exhausted, and the core of the star contracts. But, for more massive stars, the contraction causes the core to reach a hotter temperature than would a solar type star, and the higher temperature causes a new fuel to fuse: helium nuclei fuse into carbon nuclei. Now, as before, the heat generated by fusion balances the force of gravity and the star is stabilized. When one fuel is exhausted, the core again contracts, heats up and a new fusion reaction of more massive nuclei begins. There is a natural end to this process: when iron is the product of the fusion, no further fusion can occur because there is no net energy created to heat up the gas further. There is no new way to balance gravity in the core and it collapses rapidly. Eventually the electrons and protons merge and only neutrons are left. The resulting neutron star is tiny and the upper layers of the star being pulled in by gravity rebound off the hard surface of the neutron star. A massive shock wave forms that sends material flying out at very high velocities of at least 10,000 kilometers per second in a huge explosion.

The layers of the star in which fusion occurred previously are dispersed into the interstellar medium. The ejected material is so hot that new elements form from fusion or fission. The expanding stellar material can often be seen for hundreds of thousands of years. This ejection phase forms most of the elements in the periodic table heavier than iron. Most of the heavy elements lighter than iron are formed in the nucleosynthesis described in the previous paragraphs. Iron itself comes from a different type of supernova.

The stellar material expanding away from a compacted core is referred to as a supernova remnant. X-ray spectroscopy reveals the extreme heat created by a supernova shock wave in the early days of expansion and gamma ray spectroscopy reveals the radioactive decay of elements that were created during expansion. The elements created by the fusion of the burning star over its lifetime and the additional elements created during the supernova expansion created all of the elements in the known Universe, except for hydrogen, helium, deuterium and a bit of lithium. The hydrogen and deuterium must be formed in the Big Bang. The helium and lithium we see is partially from the Big Bang and partially from stars.

Gravity plays an important role in adding complexity to the Universe. The early Universe was made up of mostly hydrogen, and more complex elements would have never formed if not for gravity forcing protons to fuse together to build more complex elements. Planetary nebula and supernova remnants are some of the mechanisms that send the elements cooked-up in stars back into the Universe to create planets that contain the building blocks for life.

Compact objects

The compacted core of the star is left in the wake of a supernova explosion. The cores of stars that were more than eight times larger than the mass of the Sun are thought to become neutron stars. Neutron stars are small, but they are incredibly dense, creating a gravitational field so powerful that material accreted from a donor star emits in the X-ray waveband. Neutron stars are typically about 1.5 times the mass of the Sun, but have radii of only about 10 km.

The cores of the most massive stars can form black holes. The incredible size and density of black holes creates a gravitational field so strong light cannot escape from the surface. The intense gravitational field of a black hole can pull in stars, as well as neutron stars and other black holes. Stars that are pulled into black holes are shredded apart sending out tremendous amounts of energy and dust that collects into a hot glowing accretion disk that spins around the black hole.

Galaxy Formation

Galaxies form from collapsing dust clouds that fill large volumes of space and are often centered on extremely massive black holes. The black holes of interest are over a hundred thousand times more massive than the black holes created in the collapse of supernovae. It was only recently realized that most galaxies have massive black holes within them. The actual interaction of the dust and gas and stars with the black holes is not understood, nor is the origin of the black holes. They could be mergers of stellar-mass black holes over cosmic time or they could be primordial, resulting from an unknown process.

The gas clouds fragment and eventually make up galaxies containing millions to billions of luminous stars. Some galaxies last for billions of years. The most massive stars formed over the life of the galaxy eventually form supernovae. The gas from the supernovae is continually recycled, and gives birth to generations of new stars and planets.

Gravity is the long-range force that can pull entities with mass together over great distances to form galaxies, stars and planetary material. These objects are all the consequence of atoms and ions being first clustered into huge clouds of gas. There is observational evidence of the existence of an entity that does not radiate energy similarly to ordinary matter: this material, dark matter, does not interact in a detectable way with normal particles. The mass of this dark matter exceeds the mass of atoms and molecules in the Universe by an average factor near six and is critical to the above-noted clustering of gas into large clouds that eventually leads to the formation of galaxies, stars, and planets. Gravity is an attractive force that depends only on the mass of the constituents involved and the distance between them, not on their charge. Dark matter and regular matter seem to obey the same laws of gravity. Dark matter is not otherwise detectable as far as astronomers know, but ordinary matter is easily detected by the light it emits.

3D Map of the Universe: The Universe is very uneven: on the largest scale (left hand image) massive clusters of galaxies form what is known as a cosmic web, with enormous gaps, known as voids, where nothing seems to exist. The voids in the structure are typically 100 million light years across. The right hand image shows a greatly expanded scale, revealing that individual galaxies within the cosmic web are separated by large gaps, again with nothing in between. Such a structure is called hierarchical. Astrophysicists can't observe all the galaxies in the Universe, but they can build simulations that show structures consistent with the images in this figure.

Astronomers infer, from the existence of a faint glow of microwave radiation all over the sky, and from studying the abundances of the elements throughout the Universe, that the early Universe was an expanding, hot ionized plasma with no neutral atoms. The isolated galaxies we see today which formed out of that expanding gas are thus moving away from each other.

The early Universe hot plasma cooled into clumps of neutral material, made mostly of hydrogen atoms, and clustered around the dark matter. Those clouds would grow so dense that they would collapse under their own gravity. Many key components of the gas we see forming stars today was probably not present for the formation of these first stars. Astronomers don't know for sure, therefore, how the first stars were formed.