Day 7 The First Stars In The Milky Way

‘Day 7’ 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.


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How Many Galaxies In The Universe?

Cover: Astronomer Gerard Bodifee with wife TV Presenter Lucette Verboven

Day Six. By this stage in the life of the Universe (11,500,000,000 years ago) there exists something of the order of 170,000,000,000 (170 billion) galaxies each containing between tens of millions and billions of stars.

The name ‘galaxy’ is from the Greek word for milk which is how they described the whiteish appearance of the band of light we know as our home galaxy. Each of the 170 billion galaxies is indeed a group of stars, but much more than that. They almost certainly hold billions of planets, star systems, star clusters and in most cases, a massive black hole near the centre. They also contain a lot of material in the form of vast clouds of gas and dust particles that are mostly the remains of stars that have exploded at the end of their lives.
It has become clear that every galaxy has more mass (the ‘weight’ of something on earth) than we can measure by adding up what we can see. (A cannon ball in space may be weightless but it would still make a mess if it hit you because it still has the same mass.) This non-reflective material is predictably called ‘dark matter’ which incorporates ‘dark energy’. As Einstein showed, matter and energy are different forms of the same thing.

Galaxies come in a range of sizes and types from the dwarf galaxies with only 10 million stars to the big boys that can have one hundred trillion (that is, a hundred million million). 100,000,000,000,000. That’s a lot of zeros and it’s hard to imagine a number so large, especially when we are talking about very hot objects the size of our Sun, which itself is a million times bigger than the earth.
In between there’s not a lot going on, but technically, intergalactic space is not empty, but just as near to it as all get out, maybe 1 atom per cubic metre, which is sparse by anyone’s standards. In the popular press, quoting distances is done in light years, however the scientists in this field tend to use the parsec, which is 3.26156 light years. One might be forgiven for thinking that it’s not exactly a round number, an easily remembered number or one that trips lightly off the tongue, so why use it?
When we see a star, at the same time we also see stars further away. Six months later, when we are on the opposite side of our orbit around the sun, that particular star will appear to have moved slightly compared to the background stars. That’s because we are looking from a different angle.
You can close one eye and look at something close, then look through the other eye and it seems to move against the background. The parsec number is defined by measuring this angle. If it appears to move by 1/3600th of a degree, that is one parsec (like 3,600 seconds in an hour) the light would take 3.26156 years to reach us. The cosmologists need really big numbers and even light years aren’t big enough for the job, so parsecs it is.
Over the years, a fair number of galaxies have been catalogued, tens of thousands in fact but here is the interesting bit, there are 5 main catalogues and they don’t have all the same galaxies listed. Even when they are, they have different identification numbers.

Take Messier 109 for example. In the Messier catalogue, as you would expect, it’s number 109 but in the other catalogues it’s also code number NCG3992, UGC6937, CGCG 269-023, MCG +09-20-044 and PGC 37617. Clever boys.
Now, in the rest of the scientific world, the custom is to assign a name to whatever is being studied, even the least significant, invisible to us, microbe. Here we have entire galaxies, billions of times larger than our whole planet and all they get is a number. Well, up to five numbers actually, depending on how many catalogues we are looking at.

It should therefore come as no surprise that someone thought the galaxies were not getting a fair deal and decided to make a new catalogue, called with great imagination, ‘The Catalogue of Named Galaxies’ to give them a bit of dignity and some spiffy names.
Belgian astrophysicist Gerard Bodifee and the classicist Michel Berger began their new catalogue with one thousand well-known galaxies. Each were given a meaningful and descriptive name using the methods of other sciences like biology, palaeontology and anatomy. For example the aforementioned Messier 109 became the new ‘Callimorphus Ursae Majoris’.
Hell yeah, that’s a much better name.

One cannot pursue the subject of galaxies without mentioning ring galaxies, unusual in that they don’t seem to have much in the middle. They are thought to form in a type of bull’s eye collision when a smaller galaxy passes through the middle, cleaning up so to speak, leaving a neat ring of stars behind.
It looks like that might be what happened to the Andromeda Galaxy, perhaps more than once as it displays a multi-ring structure. There won’t be many rings left after the next collision when our own massive Milky Way smacks into it head on, which it is scheduled to do soon. (Soon in galactic terms, about 200,000,000 years from now.)

Despite the prominence of our galaxy and the millions of other large and very large galaxies, in sheer numbers, dwarfs rule. In fact you could say most galaxies are the smaller variety, perhaps only one hundredth the size of our Milky Way and often host to fewer than a hundred million stars.
Some of these are so small, light photons only take 326 years to travel from one side to the other, almost like crossing the street in galactic terms. They have all the same shapes as their larger cousins, elliptical, spiral and irregular, although the dwarf ellipticals require a little imagination to classify them that way.

Checking out our neighbourhood reveals that twenty seven of the local galaxies are dwarfs and that in itself is somehow surprising, but better yet, the mass of each dwarf (equal to about ten million of our Sun) seems to be very similar, regardless of the number of stars. This, it is argued, lends weight if you pardon the pun, to the conjecture that dark energy/matter is indeed the dark horse in gravitational theory.


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The Milky Way

Dear Diary. Day Five. After an unimaginable time span of 1,800 million years after the ‘Big Bang’ stars in this area ignite forming the Milky Way, our home galaxy.

It’s somehow comforting to think we have neighbours, perhaps lots of them, in our locality. Our “town” in the Universe is called the Milky Way and as far as we know, there are between 200 and 400 million suns much like ours in town. Now 200 million stars is a big difference, but you can’t just count them.
The main problem is that our solar system is a fair way out of town, on one of the big avenues (the Orion–Cygnus arm) and there are three others just as big. (Actually we are not right on the avenue, more like a side street off one of the main avenues.)
On top of that there are all the other stars around the centre as well so scientists have to work out the approximate mass of the galaxy and divide the answer by the average size star and you get a rough idea of how many stars there are in town.
When you think that our solar system has 8 planets made up from the leftovers from the sun’s birth, it’s hard to imagine all those other suns out there don’t also have at least a handful of planets too. To add to the fun, recent data from the Kepler space mission points to planets that are not attached to stars, just wandering about, probably a couple of hundred million of them.

Getting back to planets doing the right thing, the data strongly suggest that there are up to 40,000,000,000 planets orbiting stars in the habitable zones and 11,000,000,000 of those look just like our Sun. This is just in our galaxy so all that adds up to a lot of neighbours, but don’t expect a visit tomorrow. The nearest star to us (other than the Sun obviously) would take more than four years to get to and that’s only if we can work out some way to travel at the speed of light and we don’t bump into a speck of dust or something a little larger. The closest one that might have an earth-type planet is 12 light years away.
The reason the neighbourhood has a milky look about it is that our vision has only evolved to help us find things to eat and avoid others that might want us for lunch. Our eyes did not evolve to see stars, which is why we can only see about 10,000 of them (all in the Milky Way, although some argue Omega Centauri is just outside our galaxy) meaning we can see one star in 40,000. (An exception is the temporary super-bright flash of the death of a star, a supernova). The light from the rest blends into the band of light we see on dark nights. The dark patches are caused by interstellar dust that masks the light from the stars. In the Southern hemisphere, where the dark patches are most prominent, one of the most famous is the emu, close by that Australian icon, the Southern Cross.

It must have come as quite a surprise to Galileo, to see so many stars when he put his telescope up to his Mark-1 eyeball in the year 1610. He was the guy who worked out the earth was not the centre of anything and got belted up by the Catholic Church for saying so. All the way up to the 1920’s scientists thought the Milky Way was the only show in town. Man, were they wrong, and by a margin that’s impossible to grasp. There are literally billions of other galaxies out there (170 billion to put an approximate figure on it) most of them holding between millions and billions of suns and you can’t even see one star with the naked eye, only a few distant galaxies of stars.

Our galaxy is a spiral, that is, a centre disc with 4 major arms and fairly big as galaxies go, nothing like the real big ones but not a tiddler either at 120,000 light years across. If you thought of it as a very big city 120 kilometres across, our suburb is 27 kilometres out of town on the Orion–Cygnus arm. To get our size into perspective, if our Solar System was one inch or 25mm across, the Milky Way would be about the size of China, the USA, Australia, Canada or Brazil. As we see it from Earth, orbiting the Sun and rotating every 24 hours, the Milky Way passes overhead twice a day.
In downtown Milky Way, you will find “Sagittarius A star”, a supermassive black hole, perhaps a reminder of a few cities you’ve been to. You wouldn’t want to visit this one. It’s about 4.5 million times heavier than the Sun which is about a million times bigger than the Earth. The rest of the stars rotate around this point like a big pinwheel with the arms bending back as though they were in the wind. It takes us (meaning the Sun and our solar system) about 240,000,000 years to go around once even though we do it at a cracking pace, about 220 kilometres every second so it’s a rather long way around.
What is really weird though, is one would imagine that the further out a star is from the centre, the faster it must be travelling, however that is not what scientists have found. Most stars are moving somewhere in the 210 – 240 kilometres per second range regardless of their proximity to the middle. This seems to provide evidence for unseen matter, or dark matter as it has been dubbed, that is responsible for the variation in gravity needed to make this work.
Even that seems a stately pace when you consider the Milky Way itself is belting along at 600 kilometres a second on a collision course with the neighbour galaxy Andromeda which has 3 times as many stars although scientists think the total mass is not too different.
One fleetingly pleasant thought about our Milky Way is that is has a couple of bars. Apparently about two thirds of spiral galaxies have a bar or two but as you guessed you’ll never have a refreshing nip in one of these. For obscure reasons concerning the flow of gaseous material, the central collection of stars, including the black hole have formed up into a bar shaped structure that works mightily hard as producing new stars. We make on average, one new star every year.

I guess that means in another couple of billion years, we may have new neighbours popping up on newly formed planets, but on the other hand, some stars are running out of hydrogen so there are probably lots of planets being snuffed out every year too. It’s a rough neighbourhood.


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