This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

The Birth of Stars

by Dr Jamie Love Creative Commons Licence 1997 - 2011

By now you should be feeling comfortable finding your way around the night sky and proud of your accomplishments. Today (tonight?) you will learn a few more stars and constellations as well as learn where stars come from.

When perceived against the human life span, the universe and its stars seem eternal but they're not. Stars, like people, are born, live their lives and then they die. Unlike humans, the lives of most stars are often measured in billions of years!

If stars have such long lives, how can we see them age?

We can't. All we can do is observe the color, temperature, brightness and position of large numbers of stars and hope to piece together a "story" about how they all got to be that way. Of course, the story of stellar evolution, as this is called, is based upon a solid understanding of physics and the observations of stars. It would take years to teach you all the details of how we have come to understand stellar evolution but you don't need to know the details in order to understand the story. However, you should know that the "story of stellar evolution" is not carved in stone (yet). There are still some things we don't know - especially the details. I mention this not to confuse or worry you but to caution you that years from now you may learn that the story has changed - probably only a little and probably only a detail or two.

There is no disagreement or confusion about the origin ("birth") of stars. Stars are made from "nebula".

Most of space is a vacuum or to be more precise, most of the universe has very little gas or any other type of matter in it. However, there are places in space where there are considerable amounts of matter. Our Sun, and indeed our entire Solar System, is an example of a "high density" part of the universe in the sense that there's a lot of matter around here. But I'm getting ahead of myself. There are other places all over the universe where material accumulates due to its mutual attraction caused by gravity. All matter has mass and all masses are attracted to other masses by the force of gravity. Eventually this attraction creates a nebula - a tenuous (that means "thin") gas in space composed mostly of hydrogen but also usually containing smaller amounts of other elements and even molecules and grains that collectively we call "dust". The nebula will be visible to us here on Earth if there are stars in it or near it to illuminate or excited the nebular materials. Otherwise, the nebula will be dark and can only be observed by its ability to block out stars hidden far behind it.

Is there a nebula I can see?

Yes, and it's in my favorite constellation, ORION!
The Orion Nebula, also called "the Great Nebula of Orion", is about 1500 light-years-away and about 30 light-years wide.

The Orion Nebula is so big that it spans most of the area of the sky that we call ORION. Most of ORION's stars are on "our side" of the nebula so those stars are not obscured by it. However, a tiny cluster of stars called Trapezium (or theta-ORION using Bayer's system) is so far away and so bright that it allows us to easily see a part of this nebula. Indeed, Trapezium is actually in the nebula!

To find it you first find Orion's "sword", a line of stars that dip down from his belt, along his right side. His sword consists of three "stars" roughly of magnitude 6, 5 and 4 in that order when counted from top to bottom. The sixth order of magnitude is about as low as you can see with the naked eye on a clear, moonless night, so you may not be able to see the star at the top of his sword. That's OK because it's the middle "star" you are looking for. On a very clear night you can barely see a fuzzy image for Trapezium. That's the Great Nebula of Orion! It's dim but it's there.

Here's some advice to help you see very dim objects. Instead of looking directly at it, look a little bit to the side. Suddenly, almost magically, the dim image will appear as a kind of ghostly glow! This technique, called averted vision, relies on the fact that the focal point of your eye (called the fovea) is dominated by color sensors (called cones) which are less sensitive to light. By averting your gaze you focus the beam of light to an area of the eye that is dominated by white-light sensors (called rods). These are much more sensitive to light.
Try it! If you have never tried averted vision before you may find it a little strange at first. Novices tend to instinctively "inavert" their gaze the moment they see an averted image causing it to suddenly disappear! It's a natural instinct to focus an interesting image onto your fovea, but you should try to control that impulse. If you practice averted vision you'll get better at forming and understanding the image on the "non-fovea" portion of your eye (and mind).

Another nice trick is to use some magnification instrument. A pair of binoculars will bring you some detail of the Orion Nebula.

A very good telescope might produce this beautiful image (which was taken by Jason Ware ©).

The red on the left is caused by the EMITTED light of the dust itself. Nearby stars have heated the nebula to the point where it is emitting its own light. Note that this light is produced by the nebula and that's why it is emitted light, however the energy that causes it to emit the light is produced by the stars nearby (within).
Contrast that with the part of the nebula on the right side. Its bluish light is caused by REFLECTION of light from nearby stars. That's different. The size of the dust grains causes blue light to be reflected best.

Summary: the left side is red emitted light (like from the surface of Aldebaran) while the right side is bluish reflected light (like from the Moon).

It's hard to imagine how thin (tenuous) a nebula really is but here is one way to appreciate the Orion Nebula. Imagine you had a sampling device that allowed you to pass through a section of this nebula collecting all the matter in its path. Imagine that device was about 3 centimeters (slightly more than an inch) in diameter, so it collected a "core sample" of that diameter. If you swept it from one end of the Orion Nebula to the other, across all those light-years, you would collect less than 5 grams of material (roughly the mass of a US nickel). Now that's thin (tenuous )!

The Great Nebula of Orion is our closest star nursery. Inside this nebula stars are "born". The stars we see as the Trapezium are less than a million years old (younger than our genus Homo) and made from the same kinds of material which surrounds them. These are the youngest stars you will ever see! However, there are about 700 other "objects" in the Orion Nebula in various stages of star formation. Gravity continues to cause the nebula to shrink and condense into thicker (less tenuous) regions. As these masses accumulate they cause the center to heat up. This heating is caused by the pressure due to the increasing mass of the clumps of matter and the friction caused by the falling materials. As the nebula grows and heats up, it begins to emit a small amount of heat and it is called a protostar.

The next stage depends upon how much mass the protostar collects. If it doesn't collect enough mass (if the protostar stays less than 6.7% the mass of the Sun) it will continue emitting a little heat but not the energy of a true star. Such a "small" mass is insufficient to generate the nuclear reactions needed to "light up" a star.
A brown dwarf has masses less than that required to ignite a nuclear reaction but too large to be a planet. They are a protostar that never grows into a proper star.

Is Jupiter a brown dwarf?

Well, some people refer to Jupiter as a brown dwarf, but others insist that a brown dwarf must be over 10 Jupiter masses to earn that title. It's a matter of how you split the definitions. Jupiter has only 1/60th the mass needed to become a star. Regardless, these low mass objects will never experience life as a true star.

On the other hand, if a protostar collects sufficient mass (more than 6.7% that of our Sun, or 60 times more massive than Jupiter) the protostar goes on shrinking and heating. This causes a series of complex and irregular fluctuations in its heat and light output. These "birth contractions" go on for millions of years. This stage of stellar evolution is called the T Tauri stage - named after a very dim "fetal star" called T in the constellation of TAURUS.
[It's slightly above and to the left of Aldebaran. Don't bother looking for it because its magnitude is only 8.3 at its brightest and it fluctuates to as low as 13.5! It isn't important to find this historically interesting fetal star but it's nice to know where the name "T Tauri" came from. ]
Observations of T and other T Tauri stars show us that during this important stage in a star's development its energy fluctuations cause a huge "stellar wind" to blow away the outer layers of its nebula. Large protostars or "protoplanets" are not blown away but this wind is strong enough to strip the atmospheres off of any nearby worlds.
Long ago, when our Solar System was forming, our Sun went through the T Tauri stage and blew the atmospheres off of all the inner planets! The Earth and Venus were once as "naked" as the Moon is today. Eventually the Earth and Venus accumulated a new atmosphere from internal sources (volcanic eruptions) and external sources (comets).

The Great Nebula of Orion contains many T Tauri stars undergoing chaotic fluctuations that are working towards becoming an "adult star". With each fluctuation and contraction the interior of the T Tauri star heats up. When the core temperature reaches 10 million degrees (Celsius or Kelvin) hydrogen nuclear fusion begins and a true star is born.
[You'll learn more about this later in this month's lessons.]

Is this now a stable star?

Yes. As a matter of fact it is very stable. Stars will "burn" their hydrogen for billions of years. This brings us to another important definition and another part of our stellar evolution story. When a true star is born it starts its "life" with the fusion of hydrogen in its core.

ALL Main Sequence stars fuse hydrogen to helium in their core.

Therefore, when a protostar moves through the T Tauri stage and finally "ignites" its core with nuclear fusion, it joins the Main Sequence.

The exact position of the star along the main sequence depends upon its mass because that influences its brightness (luminosity) and surface temperature (star-type).

Rigel formed from a very large nebula so it has a great deal of mass and is called a blue giant. It's found in the upper left side of the diagram.
Our Sun is to the right-of-middle of the Main Sequence because it was formed by a small-to-medium size nebula. It joined the Main Sequence (began fusing hydrogen ions in its core) about 5 billion years ago and is not quite middle age.
At the other extreme, Wolf 339 condensed from a very small nebula to form a very small, cool star classified as a red dwarf. It had just barely enough mass to become a star and it is found in the far lower right of the diagram.
Find those three stars on this H-R diagram and think about the relative sizes of the nebulas needed to make some of the other stars I've labeled.

And any protostar that accumulates less mass than a red dwarf will be a brown dwarf.

Yes, assuming it is 10 times bigger than Jupiter (according to some definitions ). Certainly Earth is NOT a brown dwarf.

In this lesson you've learned where stars come from and you should review this information.

Stars are created from nebulas - a thin expanse of material. If the nebula is near a star or group of stars, it can be seen as emitted light (caused by re-emitting energy absorbed from the star light) or as reflected light (bounced off the nebula material). The Great Nebula of Orion displays both types. A nebula far from any stars is not directly visible and can only be detected as it blots out stars behind it, forming a "hole in the heavens" as one astronomer (William Herschel) once described them. There's no good example of such a "hole" in the Northern Celestial Hemisphere, but we will come back to this subject in a subsequent lesson about the Southern Hemisphere's night sky.

Gravity causes nebulas to clump together eventually creating large bodies that generate a small amount of heat. The heat is caused by the pressure and the friction of material falling onto it - both due to gravity. These bodies are called protostars and their fate depends upon how much more mass each can collect from the nebula. If it never reaches a mass of more than 6.7% that of our Sun it will never ignite its core with nuclear fusion so it will forever be a brown dwarf.

On the other hand, a protostar might accumulate sufficient mass from its local nebula to cause nuclear fusion to occur at its core, thus becoming a star. This "birth" of a star is not a straightforward process and it involves a great deal of contractions and false starts as it develops. This is called the T Tauri stage and is characterized by wild fluctuations in brightness and energy output over millions of years. In the process a layer of the outer shell of the "fetal star" is blown away by a powerful "stellar wind". Eventually stable nuclear fusion takes hold in the core and a star is born.

The star will become part of the Main Sequence. Its exact position on the H-R Diagram will depend upon its total mass.
Because stars start inside nebulas it's understandable that a young nebula, without any stars in it, will be of the dark variety, while an older nebula, old enough to have had time for stars to form, will often be illuminated by its "children". This group of children form a star cluster.

Star cluster? What's that?

That's our next topic. Feel free to take a break, otherwise, continue on to your next lesson and learn about star clusters.




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.