The first Milky Way stars. Day Seven. The first stars in the Milky Way are now a thousand million years old.
Aside from the twinkling variety, how many types of stars are there? Seems like a fair question and the answer is a very scientific one too, ‘lots’.
Actually, most of the stars are not alone with twins being the most common setup in most galaxies and a fair helping of triplets too. This is not surprising when you know how the stars get going in the first place.
It is fairly widely known that stars form from clouds of gas but we tend to think of this as a ‘one cloud – one star’ event. In reality, the clouds that start the process are so vast, as they collapse under the force of gravity, the core is usually about 100 times more massive than our Sun. This core then usually fragments into smaller clumps, each one with the potential to become a star. (No, not like an audition).
These protostars will start out 10 to 50 times bigger than the sun depending on how many form out of the cloud core and his happens fairly quickly, perhaps 10 million years or so. (If you’re a universe this is positively scooting along.)
As gas is pulled into the core of each one, the temperature rises to a few thousand degrees and infra-red radiation is released, which is how astronomers can ‘see’ them (no light at this stage). Eventually pressure in the core puts up the ‘no vacancy’ sign and the balance between pressure outwards and gravity inwards reaches an agreement. The core is only about 1% of what the star will become once it moves through the proto stage.
Gravity is still doing its thing and pulling in more gas but the core is full, so it builds up putting more pressure on the core. When the core becomes hot enough to begin nuclear fusion, the stellar wind created pushes back against any new material being gathered by gravity and it is now considered an operational protostar.
The next step for our baby star (and probably its brothers and sisters ‘nearby’) is to start organizing its stellar wind along the rotational axis that is mainly flowing out at the poles. Material excreted forms huge discs and material then tends to fall back to the ‘surface’ and begin to glow. A part of this material is ejected far enough to begin clumping into balls, aka planets either the heavy rocky kind closer to the star or the gas type further out.
This early period is called the T-Tauri phase, a ‘type’ of star and later as it heats up it becomes another ‘type’ of star, a main sequence star. Our Sun is in the main sequence stage. In fact most of a star’s life is spent in the 10,000,000,000-year long main sequence phase. The length of time spent in the T-Tauri (young bull?) phase depends to some extent on the initial size. Very massive stars don’t waste much time in the first stage and move into main sequence very rapidly.
If a really massive star forms in the cloud it can become a supernova when it dies instead of gracefully aging into the white dwarf stage. When the explosion occurs at the end, the pressure on the cloud can create many new protostars, so sometimes there are groups of young stars all in the same neighbourhood. This is the major contributor to the pinwheel effect in spiral galaxies.
At the other end of the scale we have clumps that are not big enough and just don’t make it. To become a protostar the gravity-induced hot spot has to be a minimum of about 75 times more massive than our solar system’s gas giant planet Jupiter.
A clump that is almost big enough will be, say 70 times more massive than Jupiter but its physical size will be similar, meaning a lot of gas has been compressed down to Jupiter size but still not enough to cause nuclear fusion. If you want to be a star you need to be at least 8% of the size of the Sun, the minimum requirements. If not, this is another ‘type’ of star called a brown dwarf, halfway between a gas giant planet and a star. They call it that because of its red colour. (I don’t know either.)
To put this in perspective, Jupiter is roughly the same density as the Sun which should be no surprise being essentially the same gas, 75% hydrogen and 25% helium (measured by mass or ‘weight’ but 90% -10% by volume because helium is heavier than hydrogen).
Jupiter, although all gas (or very nearly all) is 318 times ‘heavier’ than earth but our planet would fit inside 1,320 times over. The smallest star would be the same diameter size as Jupiter but 25,000 times more massive than the earth, not that you’d want this baby too close.
Even if it did not quite make the grade and had to settle for being a brown dwarf, it is still very hot and will stay that way for a very long time because being small with a surface area to match, it takes a long time to cool. Eventually though, nature will have its way and our little brown/red dwarf will fade to become a black dwarf, yet another ‘type’ of star.
The tendency of the clouds in many galaxies to produce stars in batches mean that not only do we see planets revolve around stars, many stars revolve around or in step with other stars too. This is commonly demonstrated as twin stars (binary pairs) orbiting each other in a space Methodist dance, referred to without imagination as a wide pair.
Those in a more intimate embrace tend to exchange bodily material (gas) and while not physically touching, are still referred to as ‘contact’ binaries.
While the more restrained wide pairs evolve separately, they are still married to one another by gravity. Most binaries, double stars to us, appear close to each other and we could assume both about the same distance from us, but in reality one can be much further away only appearing to be close from our perspective.
Only very occasionally, can we find a pair that orbit each other on an ‘edge-on’ plane that aligns with our position so we can see a change or dimming of the light as one passes in front of the other from our vantage point. Nonetheless, twins and triplets are the most common form of star in most galaxies like our own Milky Way.