All stars at the beginning of their lives start off as molecular clouds of dust and gas. Something will cause the gas to collapse in on itself and begin the process of turning into a star. The cause of the collapse could be a passing star that disturbs the cloud or an explosion from a supernova. Our Sun was once a molecular cloud many billions of years ago.
Another way to call a Protostar is a Pre-star, not yet a star but has moved on from being just a cloud. Imagine the life of a star as a person. If a fully grown star is an adult, a Protostar is a child, one that is still growing and trying to find its way in the Universe. The Protostar is not yet an adult or in the Hertzsprung-Russell diagram, a main sequence star. Whilst a child phase may last only for the first 18 years of its life, a Protostar may be in this stage for anything between 100,000 and 10 million years.
When the molecular cloud has begun to collapse, it creates a Protostar, one that is not yet started nuclear fusion, that is to turn hydrogen into helium because it is too small and doesn't have enough mass to which to generate the necessary fusion. A Protostar needs to get to a temperature of 10 million degrees kelvin for fusion to occur. Utah University.
The heat from the Protostar is generated as the gas collapses in on itself rather than because of nuclear fusion. The nucleus of the star may be hidden from view because of the surrounding dust and cloud. One such Protostar is V1647 Orionis which I probably don't have to tell you but its in the constellation of Orion.
As the Protostar gets going, its magnetic field becomes stronger. As the magnetic field becomes strong, it can create a solar winds that blow away the excess dust and gas. These expulsions can be in the form of streams. As the dust is pushed away, the Protostar can become visable. See Las Cumbres Observatory for reference link. The picture below from HubbleSite shows a binary protostars called LRLL 54361 in the constellation of Perseus. As you can see from the artists impression, the two stars are in orbit round one another. The Protostars are in the star-forming area known as IC 348.
Although the mass might be on its way to being a star, it is still dark from the outside so we are unable to see it clearly. The stuff that doesn't get pulled into the Protostar can and will possibly start coalescing and could form planets. Our solar system was formed from a molecular cloud, the inner cloud became the Sun and the other gasses the planets. Thought
The collapsing star could create a binary star system or as in our case planetary objects. The material in our bodies existed one time in the molecular cloud but fortunately, the moleculars weren't pulled into the star but was able to exist to create you and me.
After the Protostar stage, a star can go down one of four phases, T-Tauri, Main Sequence, Brown Dwarf or a Gas Giant.
For some protostars that are less than three Solar masses, the next stage is T-Tauri stage where the star has blown away the dust but is not yet able to start nuclear fusion. A T-Tauri star will be in this phase for something like 100 million years. The star will be larger than a normal star and will eventually reduce in size to become a Main Sequence star.
Some Protostars can fail to become a star like our Sun. When they fail to become a star, the can become Brown Dwarfs and Gas Giants. Both types are large balls of gas to put it one way and not too dissimilar from one another.
A star that doesn't have enough mass to generate the necessary heat to become a star can become a brown dwarf. An example of a brown dwarf is 54 Piscium B, a brown dwarf that is orbiting its parent star. Brown dwarf stars are hard to spot given that they are brown and therefore do not stand out in the darkness of space. Not many of them are know. 54 Piscium B is presumably known because it is located next to a main sequence star.
Although brown stars can't fuse hydrogen, they are able to fuse Deuterium which is a rare form of Hydrogen. They all have a single proton but Deuterium will have an additional neutron. Just for measure, a Tritium will have two neutrons. Lawrence Berkeley Laboratory
Brown dwarfs are very hard to spot due to them being dim compared to other stars. It is believed that there might actually be more brown dwarves out there than actual stars. Las Cumbres Observatory
The two largest gas giants in our solar system could one day have been a protostar but never gained the necessary mass to start nuclear fusion. They will not spontaneous start fusion all of a sudden because they are not growing sufficiently.
One of the smallest stars known, EBLM-J055-557 AB is a star that is actually smaller than Jupiter. The reason why EBLM exists as a star whereas Jupiter is only a gas giant is because EBLM has more mass and therefore kicked off the fusion.
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