How long does it take a star to form?

Does a star form over a long period of time, or does it form quickly? Would a new star slowly begin to brighten or would it be like flipping on a light switch? Do stars grow during their lifetime by consuming matter, or are they more or less their final size when born?





I realize the actual process of forming a star probably take a long time, I more interested in what happens once it has collected enough matter/mass to ignite.|||Stars start off as giant nebulae of very cold (%26lt;10K) gas. I am sure you have a basic understanding of how these clouds collapse so I shall try to answer the specifics you asked.





No, when they are born they are tiny. Very tiny. A usual core forming in the middle of the cloud can being the process of fusion when there is only 1/5 times the mass of our sun, and then spend a much longer time collecting the rest of their mass to become a newborn fully fledged star.





Here is a little more detail about how it reaches that stage. What I am about to describe takes about 1 million years. When the collapsing cloud reaches 10^-13 grams per cubic centimeter (about 20 billion hydrogen molecules per cm^3), it becomes dense enough to trap infra-red radiation from within. This allows it to start warming up. It continues to contract, but slows as the pressure of heat starts increases, and comes to a halt when it reaches about 2000 times more density (about 40,000 billion H2 per cm^3). The core now has a mass of roughly one hundredth of a solar mass, and extends over a volume of several AU.





When the temperature inside reaches about 2000k, the molecules are broken into constituent atoms and a new phase of collapse occurs, and a new "inner" inner core is formed. This stops collapsing when it reaches about 10,000K, and the atoms become a plasma. It become opaque to radiation and this is the point when it is refered to as a star.





Its size doesn't change very much from now on - it is about the size of the sun today - but its density - at the moment about on thousandth of the sun - is what will continue to increase. All of the gas of the first core takes about 10 years to fall into this protostar, bringing it up to about 0.01 solar masses. Then the rest of the cloud continues to fall into it bringing its mass up to its final mass. It will grow during this period to about 4 times the radius of its final size, but slowly shrink to its final size over 10 million years or so. Most of the energy it released until this time was due to the gravitational potential energy it released as it collapsed, but eventually it settles in the nice balance of gravity and pressure from the fusion of its core.





As for when it "lights up". It never really does so. It becomes brighter slowly over time, and observers from outside will hardly notice due to all of the dust surrounding it. When fusion begins it will become noticeably brighter in a short time, but due to the surrounding gas, it will still take millions of years to reach its full glory.|||People have seen protostars inside dust clouds start to glow in infrared. But from a great distance, it is hard to judge the exact time of start of fusion, the "ignite" time you are asking about.


If accretion continues at rates of 10^-5 to 10^-3 solar masses per year, it is not until about 40 solar masses (taking from 400,000 to 4000 years) to form the nebula around the birth site, will thermonuclear ignition happen.


A new star will blow away a lot of gas and dust from radiation pressure, losing a lot of mass until gravity and internal pressure reach an equilibrium.|||Before the star ignites, it will have already started glowing in the IR from the release of gravitational energy as it contracts (protostars, and the Herbig-Haro objects associated with star-birth regions). Once it ignites, it will take a long time for the first photons from its center to make their way to the surface (in sun's case, a million years on the average); so slow brightening would be more of a case, spread out over thousands of years, as nuclear reactions spread throughout the central part of the star and the number of photons released increases until equilibrium is reached.|||Approximately 10 million years. Really, it depends on what it's spectral type.