Astronomers classify the stars out there into groups based on the color of the star and the presence of certain elements in the star’s atmosphere. The classifications are: O, B, A, F, G, K, M (just remember this handy mnemonic , “Oh be a fine girl, kiss me”.) F stars are still hotter than the Sun, appearing white to our eyes.
F stars have a surface temperature of 6,000 Kelvin to 7,200 Kelvin. You can also recognize an F star by the presence of Calcium in their spectral signature, as well as neutral metals like Iron and Chromium. F stars represent 3.1% of all stars.
Some familiar F stars include Arrakis, Canopus, Procyon.
We have written many articles about stars here on Universe Today. Here’s an article about some strange observations of Procyon.
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Astronomers have developed a star classification system to organize all the stars we can see in the Universe; it’s based on color and the spectral signature of certain elements in the star’s atmosphere. The classifications are: O, B, A, F, G, K, M (here’s a handy mnemonic , “Oh be a fine girl, kiss me”.) A stars are some of the more common stars seen with the unaided eye: they appear white or bluish-white.
The surface temperatures of A stars range from 7,400 Kelvin to 10,000 Kelvin; that’s about twice the temperature of the Sun, so these stars are really hot. Astronomers also recognize them by the strong hydrogen lines, as well as lines of ionized metals, like Iron, Magnesium and Silicon. A stars are more massive than the Sun, but don’t lead lives that are too much different than the life of our own Sun.
Some familiar A stars include Vega, Sirius, and Deneb.
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Astronomers have developed a method of classifying stars based on their color and some other characteristics. The star classifications are O, B, A, F, G, K, M (you can remember that with the handy mnemonic, “Oh be a fine girl, kiss me”.) O stars are the most extreme group of all. They have the highest temperatures, the most luminosity, and the most mass (oh, and the shortest lives).
An O star appears blue to the eye, and can have a surface temperature of more than 41,000 Kelvin; its color would be better described as ultraviolet, but we can’t see that color with our eyes. The surface temperature of an O star is so great that hydrogen on the surface of the star is completely ionized, but other elements are more visible, like Helium, Oxygen, Nitrogen, and Silicon.
O stars are very massive and evolve very rapidly. Shortly after they form as a protostar, they already have the pressure and temperatures in their cores to begin hydrogen burning. The O stars light up their stellar nurseries with ultraviolet light and cause the clouds of nebula to glow. You can thank O stars for illuminating the beautiful nebula photographs captured by Hubble. O stars burn through their fuel quickly, and can detonate as supernovae in just a few million years.
Some O stars include Zeta Orionis, Zeta Puppis, Lambda Orionis, Delta Orionis.
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Although they’re just hot balls of hydrogen and helium, stars are constantly changing over time. Studying star evolution is a whole branch of astronomy, and scientists are learning new things all the time.
To really understand star evolution, you’ve got to go right back to the beginning. All the stars we see today started out as large clouds of cold molecular hydrogen. Some event, like a nearby supernova, passed through the cloud of gas and gave it the kick it needed to begin collapsing. The gravity of the cloud pulled unevenly, and so it tore into smaller clouds, each of which would go on to form a new star.
In one cloud, the material streamed together to form a growing ball of hydrogen and helium. This protostar was enshrouded in gas and dust, and would actually be invisible from our Earth-based telescopes. As the ball grew, more and more material came in, causing the protostar to spin, and releasing jets of material from its poles. This accumulation of material takes about 100,000 years.
Once all the material was accumulated, the pre-star was hot and glowing; almost like a real star. But it wasn’t heated by fusion reactions in its core, but through the gravitational energy of the continuously collapsing material. This hot, young object is known as a T Tauri star, and remains in this state for about 100 million years.
Finally the temperature and pressure at the core of the star were sufficient to allow nuclear fusion to get going. Now the star would become a true main sequence star, converting hydrogen into helium at its core. A star with the mass of our Sun could stay in the main sequence stage for more than 12 billion years. More massive stars will last for shorter periods of time, while the tiny red dwarf stars can last for hundreds of billions and even trillions of years.
Eventually the star runs out of hydrogen fuel in its core. Without the outward light pressure from the fusion reactions, the star starts to contract, creating more temperature and pressure in the core. A shell of hydrogen around the core can now undergo nuclear fusion, and so it does, increasing the star’s brightness hundreds and even thousands of times. And in the core of the star, helium is fused into even heavier elements. This causes the star to bloat out to become a red giant. Regular stars like our Sun will expand to the point that they consume the interior planets: Mercury, Venus and even Earth. Stars with more than 20 times the mass of the Sun become red supergiants, expanding out more than 1500 times the radius of the Sun. Imagine a star so big it consumed the orbit of Saturn!
This extra fuel runs out and so the star collapses down on itself again. More massive stars will be able to do this trick multiple times, burning new shells and burning heavier and heavier elements. Eventually all stars reach their limit. The most massive stars, those with more than 20 times the mass of the Sun, will detonate as supernovae. Less massive stars will eject their outer layers and then collapse inward forming a white dwarf, neutron star or black hole. Our Sun will form a white dwarf; a remnant the size of the Earth with 60% of its original mass. Although initially hot, this white dwarf will slowly cool down over time, eventually becoming the background temperature of the Universe.
And that’s star evolution, from cloud of gas to white dwarf.
Are you ready for more stereo vision? This haunting Hubble Telescope image has been visualized for dimension by the one and only Jukka Metsavainio and gives us a look at one of the most complex planetary nebulae ever photographed. Inside NGC 6543 – nicknamed the “Cat’s Eye Nebula” – the Hubble has revealed delicate structures including concentric gas shells, jets of high-speed gas, and unusual shock-induced knots of gas… and thanks to Jukka’s “magic vision” we’re able to take a look into the Cat’s Eye as it might appear in dimension. Step inside and let’s learn more…
When Jukka produces an image, it’s more than just a clever Photoshop “trick”. Hours of study must go into each image, because the light is acting differently in each part of the nebula. To make these images work correctly, Jukka must understand which stars are causing the ionization, which stars are nearer and further from our point of perspective and so on. Each type of image is totally unique and what makes dimension work for a reflection nebula won’t work for an emission nebula. Says Jukka; “To be able to make those stereo pairs, one have to learn lots of things about the targets, and beside that, study the actual image more deeply than usual. Star distances must be measured by the size and the color. For example, stars with yellowish hue must be in or behind the nebulosity, white/blue ones are front of it.”
Because dimension will appear reversed by the method you choose to use to view these images, Jukka makes two versions. The first you see at the top of the page is parallel vision – where you relax your eyes and when you are a certain distance from the monitor screen the two images will merge into one to produce a 3D version. The second – which appears below – is crossed vision. This is for those who have better success crossing their eyes to form a third, central image where the dimensional effect occurs. So, now that you understand the images are a visualization and how they are created, let’s take a parallel look…
NGC 6543 Cross - Jukka Metsavainio
And now it’s my turn to add a little “magic” to what you see.
Estimated to be 1,000 years old, the Cat’s Eye is a portrait of a dying star – and quite possibly an unresolved double-star system. According to research, the dynamical effects of two stars orbiting one could very well be the cause of the complicated and intricate structures revealed here – structures normally not seen in planetary nebulae. When NGC 6543 was first observed spectroscopically, it showed the presence of emission lines, an indicator of multiple stars, but also an indictor of diffuse gas clouds.
As studies progressed, more hypothesis about the NGC 6543’s structure began to emerge. Perhaps a fast stellar wind from the central star created the elongated shell… It could be the companion star is emitting high-speed jets of gas that lie at right angles to the equatorial ring… Maybe the stellar wind has carved out the inner structure of the nebula are there are more than one there? Says L.F. Miranda; “The velocity field of NGC 6543 shows the existence of two concentric ellipsoidal shells in the nebula. The two shells likely represent the inner and outer surface layers of a geometrically ‘Thick Ellipsoid’ (TE) which constitutes the basic structure of NGC 6543.”
Even more research ensued, and with it came the twin jet theory and the ejection of materials spaced over intervals of time – like cosmic smoke rings being puffed off at perfect intervals. According to Bruce Balik; “Hubble archival images of NGC 6543 reveal a series of at least nine regularly spaced concentric circular rings that surround the famous nebular core, known as the Cat’s Eye Nebula. The rings are almost certainly spherical bubbles of periodic isotropic nuclear mass pulsations that preceded the formation of the core. Their ejection period is consistent with a suggestion that quasi-periodic shells are launched every few hundred years in dust-forming asymptotic giant branch (AGB) winds but not consonant with the predictions of extant models of core thermal pulses (~105 yr) and surface pulsations (~10 yr).”
To be sure, there are simply a lot of things that we don’t know or understand about the Cat’s Eye Nebula just yet. It is possible that magnetic activity somewhat similar to our own Sun’s sunspot cycle, could be at work here. Says Dr. Balick; “What do the rings imply? Since they’re larger than the bright cores of the nebulae that they surround, the rings are almost certainly material ejected episodically before the main and bright core of the nebula formed. This means that the start that ejected the nebulae first quivered and shivered and made these concentric rings. Then something big happened, and the density and mechanism for ejecting the mass changed abruptly. This is when the core of the nebula was formed, typically between 1000 and 2000 years ago. The rhythmic ringing of a dying star is expected as the last of its nuclear fuel is suddenly triggered into ignition by the increasing crush of gravity — much like the juice ejected by squeezing an orange with increasing force. Each expulsion of juice temporarily relieves the internal pressure inside the orange. Similarly, each ejection of mass temporarily stops the combustion of the final dregs of the star’s remaining fuel. Why should the pattern of ejection mass change so radically and strongly? We can only conjecture. Its possible that an orbiting star or giant planet falls onto the dying star. It hits the surface with such force that its atoms ignite in a large conflagration. Somehow, the burst of heat drives the remnants of the dying star into space in fantastic patterns.”
Satellite image of bushfires in southeast Australia taken Feb. 7, 2009. NASA image courtesy the MODIS Rapid Response Team, NASA Goddard Space Flight
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UPDATE: Satellite Images from February 9 have now been added below.
As of this writing, 94 people (update 2/9/09) 135 have been killed by out-of-control bushfires in southeast Australia. This image from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite shows multiple large fires (outlined in red) burning in Victoria on February 7. Huge plumes of smoke spread southeast, driven by fierce winds. Click here to see a larger version of the image, which shows a larger area, and a dust storm blowing over interior deserts to the northwest. News sources report these fires sprang up and exploded in size in just a few short hours. According to ABC News, authorities suspect arsonists are responsible for some of the fires. NASA says images captured by another satellite, the Terra MODIS sensor, just a few hours prior to this image showed no sign of these fires. Twice-daily images of southeastern Australia are available from the MODIS Rapid Response Team, and Universe Today will try to update the images when they are available. See more below.
Satellite image aquired Feb. 9 of southeastern Australia bushfires. NASA image courtesy the MODIS Rapid Response Team, GSFC.
The bushfire pictures above and below shows the Barry Mountains of central Victoria on February 9, 2009. The image at top is a natural-color (photo-like) view captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite. Places where the sensor detected active fire are outlined in red. The image below is the same scene shown in false color, using visible, near-infrared, and shortwave infrared light. Burned areas are brick red, and places of intense heat—often a sign of open flame in this kind of image—are glowing pink. Smoke turns a transparent blue, which makes it easier to see the ground. False color image acquired Feb. 9. NASA image courtesy the MODIS Rapid Response Team, GSFC
Southeast Australia has a history of severe fire problems, with some historic deadly fires such as Ash Wednesday of 1983, and lesser fires almost every year. The state of Victoria averages about 19 large fires (over 1,000 hectares) per year, but the fires this year are considered to be the worst ever. These fires are often fast like grassfire but more intense. 700 homes have been destroyed, and it is feared the death toll will rise to over 100. Twenty-six fires continue to burn across Victoria; 12 of those are out of control. Satellite image of Australian bushfires from January 30, 2009. NASA image created by Jeff Schmaltz, MODIS Rapid Response Team, Goddard Space Flight Center.
This image was taken on January 30, during the beginning of when some of the fires began to break out. A crippling heat wave and strong winds contributed to an outbreak of forest and grassland fires in Victoria.
Artist's rendering of the Giant Magellan Telescope and support facilities at Las Campanas Observatory, Chile, high in the Andes Mountains. Photo by Todd Mason/Mason Productions
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Astronomy organizations in the United States, Australia and Korea have signed on to build the largest ground-based telescope in the world – unless another team gets there first. The Giant Magellan Telescope, or GMT, will have the resolving power of a single 24.5-meter (80-foot) primary mirror, which will make it three times more powerful than any of the Earth’s existing ground-based optical telescopes. Its domestic partners include the Carnegie Institution for Science, Harvard University, the Smithsonian Institution, Texas A & M University, the University of Arizona, and the University of Texas at Austin. Although the telescope has been in the works since 2003, the formal collaboration was announced Friday.
Charles Alcock, director of the Harvard-Smithsonian Center for Astrophysics, said the Giant Magellan Telescope is being designed to build on the legacy of a rash of smaller telescopes from the 1990s in California, Hawaii and Arizona. The existing telescopes have mirrors in the range of six to 10 meters (18 to 32 feet), and – while they’re making great headway in the nearby universe – they’re only able to make out the largest planets around other stars and the most luminous distant galaxies.
With a much larger primary mirror, the GMT will be able to detect much smaller and fainter objects in the sky, opening a window to the most distant, and therefore the oldest, stars and galaxies. Formed within the first billion years of the Big Bang, such objects reveal tantalizing insight into the universe’s infancy.
Earlier this year, a different consortium including the California Institute of Technology and the University of California, with Canadian and Japanese institutions, unveiled its own next-generation concept: the Thirty Meter Telescope. Whereas the GMT’s 24.5-meter primary mirror will come from a collection of eight smaller mirrors, the TMT will combine 492 segments to achieve the power of a single 30-meter (98-foot) mirror design.
In addition, the European Extremely Large Telescope is in the concept stage.
In terms of science, Alcock acknowledged that the two telescopes with US participation are headed toward redundancy. The main differences, he said, are in the engineering arena.
“They’ll probably both work,” he said. But Alcock thinks the GMT is most exciting from a technological point of view. Each of the GMT’s seven 8.4-meter primary segments will weigh 20 tons, and the telescope enclosure has a height of about 200 feet. The GMT partners aim to complete their detailed design within two years.
The TMT’s segmented concept builds on technology pioneered at the W.M. Keck Observatory in Hawaii, a past project of the Cal-Tech and University of California partnership.
Construction on the GMT is expected to begin in 2012 and completed in 2019, at Las Campanas Observatory in the Andes Mountains of Chile. The total cost is projected to be $700 million, with $130 million raised so far.
Artists concept of the Thirty Meter Telescope Observatory. Credit: TMT
Construction on the TMT could begin as early as 2011 with an estimated completion date of 2018. The telescope could go to Hawaii or Chile, and final site selection will be announced this summer. The total cost is estimated to be as high as $1 billion, with $300 million raised at last count.
Alcock said the next generation of telescopes is crucial for forward progress in 21st Century astronomy.
“The goal is to start discovering and characterizing planets that might harbor life,” he said. “It’s very clear that we’re going to need the next generation of telescopes to do that.”
And far from being a competition, the real race is to contribute to science, said Charles Blue, a TMT spokesman.
“All next generation observatories would really like to be up and running as soon as possible to meet the scientific demand,” he said.
In the shorter term, long distance space studies will get help from the James Webb Space Telescope, designed to replace the Hubble Space Telescope when it launches in 2013. And the Atacama Large Millimeter Array (ALMA), a large interferometer being completed in Chile, could join the fore by 2012.
Image where PHA 2009 BD81 (left) was discovered. PHA 2008 EV5 is on the right. Image courtesy Robert Holmes.
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While observing a known asteroid on January 31, 2009, astronomer Robert Holmes from the Astronomical Research Institute near Charleston, Illinois found another high speed object moving nearby through the same field of view. The object has now been confirmed to be a previously undiscovered Potentially Hazardous Asteroid (PHA), with several possible Earth impact risks after 2042. This relatively small near-Earth asteroid, named 2009 BD81, will make its closest approach to Earth in 2009 on February 27, passing a comfortable 7 million kilometers away. In 2042, current projections have it passing within 5.5 Earth radii, (approximately 31,800 km or 19,800 miles) with an even closer approach in 2044 2046. Data from the NASA/JPL Risk web page show 2009 BD81 to be fairly small, with a diameter of 0.314 km (about 1000 ft.) Holmes, one of the world’s most prolific near Earth object (NEO) observers, said currently, the chance of this asteroid hitting Earth in 33 years or so is quite small; the odds are about 1 in 2 million, but follow-up observations are needed to provide precise calculations of the asteroid’s potential future orbital path.
In just the past couple of years, Holmes has found 250 asteroids, 6 supernovae, and one comet (C/2008 N1 (Holmes). However, he said he would trade all of them for this single important NEO discovery.
“I was doing a follow up observation of asteroid 2008 EV5,” Holmes told Universe Today, “and there was another object moving right next to it, so it was a pretty easy observation, actually. But you just have to be in the right place at the right time. If I had looked a few hours later, it would have moved away and I wouldn’t have seen it.” A map generated by Holmes showing the path of 2009 BD 81. Credit: Robert Holmes
A few hours later, teacher S. Kirby, from Ranger High School in Texas, who was taking part in a training class on how to use the data that Holmes collects for making observations used Holmes’ data measuring 2008 EV5 and also found the new object. Shortly after that, a student K. Dankov from the Bulgarian Academy of Science, Bulgaria who is part of ARO education and public outreach also noticed the new asteroid. Holmes listed both observers as co-discovers as well as another astronomer who made confirmation follow-up observations of what is now 2009 BD81.
Holmes has two telescopes, a 24-inch and 32-inch. Holmes' 24 inch telescope. Courtesy Robert Holmes
He works night-after-night to provide real-time images for the IASC program, uploading his images constantly during the night to an FTP site, so students and teachers can access the data and make their own analysis and observations from them. IASC is a network of observatories from 13 countries all around the world.
Holmes is proud of the work he does for education, and proud of the students and teachers who participate.
“They do a great job,” he said. “A lot of the teachers are doing this entirely on their own, taking it upon themselves to create a hands-on research class in their schools.” Holmes said recently, two students that have been involved with IASC in high school decided to enter the astrophysics program in college.
“I feel like we are making a difference in science and education,” he said, “and it is exciting to feel like you’re making a contribution, not just following up NEO’s but in people’s lives.”
Holmes also owns some of the faintest observations of anyone in the world.
“My telescopes won’t go to 24th magnitude,” Holmes said, “but I’ve got several 23rd magnitudes.”
“Getting faint observations is one of the things NASA wants to achieve, so that’s one of the things I worked diligently on,” Holmes continued. The statistics on the site bear that out clearly, which shows graphs and comparisons of various observatories.
To what does Holmes attribute his success? “It’s obviously not the huge number nights we have in Illinois to work,” Holmes said. The East-Central region of Illinois is known for its cloudy winter weather, when we often have our poorest astronomical “seeing.”
“However, I work every single night if it’s clear, even if it’s a full moon,” he said. “Most observatories typically shut down three days on either side of a full moon. But I keep working right on through. I found that with the telescopes I work with, I’ve been able to get to the 22nd magnitude even on a full moon night. Last year, I got about 187 nights of observing, which is the same number as the big observatories in the Southwest, when you take off the number of cloudy nights the 6 nights a month they don’t’ work around full moons. Sometimes you just have to work harder, and work when others aren’t to be able to catch up. That’s how we are able to do it, by working every single chance we have.”
He works alone at the observatory, running the pair of telescopes, and doing programming on the fly. “I refresh the confirmation page of new discoveries every hour so I can chase down any new discovery anyone has found,” he said. “If I just pre-programmed everything I wouldn’t have a fraction of the observations I have each year. I’d miss way too many because some of the objects are moving so fast.”
Holmes said some objects are moving 5,000 arc-seconds an hour on objects that are really close to Earth. “I’ve seen them go a full hour of right ascension per day and that’s pretty quick. They can go across the sky in four or five days,” said Holmes. “And there have been some that have gone from virtually 50 degrees north to 50 degrees south in one night. That’s was a screaming fast object, and you can’t preprogram for something like that, you actually have to be running the telescope manually.”
Overhead view of 2009 BD81's path. Courtesy Robert Holmes
2009 BD81 is listed as a “risk” object on the NASA/JPL website. This is the 1,015th PHA discovered to date.
“It ranks high as a NEO in general,” said Holmes, “although not in a super-high category as far as the Torino scale,” which categorizes the impact hazard of NEOs. “At this point it’s considered a virtual impactor and that is typically is as high of a rating that you get at this point.”
“Because it is a virtual impactor, it will remain on that webpage and ask for observations every single night until it is removed as a virtual impactor or becomes too faint to see,” said Holmes. “In the past year, we’ve removed 23 virtual hazardous objects, which means there have been enough observations that the orbit of that object is no longer considered a threat to our planet.” 2009 BD81. Courtesy Robert Holmes
Because of the small number of observations of of 2009 BD81, the current chance of it hitting Earth is small. “The odds are really small right now,” said Holmes, “however, the smaller your orbital arc is the wider the path is at that point is of potential impact. The longer the arc gets, the narrower the cone of opportunity of impact becomes, and once that cone is no longer pointing at earth in the future, it is removed as a possible impactor.”
Holmes said the excitement of this discovery has been exhilarating. “It’s been a lot of fun. The energy level gets pretty high when you have something like this show up,” he said. “It’s pretty rare, and this is the first time I’ve ever had a NEO discovery. I’ve had several hundred asteroids, and just since the beginning of the school year we have had about 40 asteroids that students and teachers have discovered in the program. So having this as a NEO is kind of a nice thing.”
Holmes said he’ll track 2009 BD81 as long as he possibly can.
Holmes previously was a commercial photographer who had over 4,500 photographs published worldwide in over 50 countries. “At first astronomy was just a hobby in the evening,” said Holmes. “I worked with schools, who used the data and made some discoveries of supernovae and asteroids. It came to a point where it was really hard to work all day as a photographer and work all night in astronomy getting data for students.” So, he chose astronomy over photography.
Holmes now works under a grant from NASA to use astrometry to follow-up new asteroid discoveries for the large sky surveys and help students look for new asteroid discoveries for educational outreach programs.
One would assume that as a former commercial photographer, Holmes would attempt to capture the beauty of the night sky in photographs, but that’s not the case.
“The only thing I’m really interesting in is the scientific and educational aspect of astronomy,” said Holmes. “I’ve never taken a single color, pretty picture of the sky in the half a million images I’ve taken of the sky. It’s always been for research or education.”
Holmes is considered a professional astronomer by the Minor Planet Center and International Astronomical Union because he is funded by NASA, so that means he wasn’t eligible to receive the Edgar Wilson award when he found a comet last year.
Because of Holmes outstanding astronomical work, he is also an adjunct faculty member in the physics department at Eastern Illinois University in Charleston, Illinois.
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When a star is still in the earliest stages of formation, it doesn’t have enough temperature in its core to ignite fusion of hydrogen and helium. Instead, the star shines with just the gravitational energy of its continue collapse. Astronomers call this pre-star a T Tauri star. This early stage lasts about 100 million years before nuclear fusion kicks in and it becomes a true star.
As you might know, stars start out as vast clouds of cold molecular hydrogen. Some event, like a nearby supernova, causes the cloud to collapse. As it collapses, the cloud fragments into separate pieces, each of which will eventually become a star. The first stage in a star’s life is as a protostar. This is where fresh material is still falling into the center of the collapsing cloud. After about 100,000 years or so, everything is collected, and the T Tauri star gets going.
T Tauri stars actually look quite similar to main sequence stars. Their surface temperatures are about the same as a star of a similar mass, but they’re more luminous because they have a larger diameter. But T Tauri stars get all their energy from the gravitational collapse of the material. They’re violent babies. There’s evidence that T Tauri stars are covered with active sunspots, and they produce extremely powerful stellar winds. Some of the brightness we see from here on Earth is the T Tauri star’s powerful stellar winds heating up a protoplanetary disk surrounding them.
Over 100 million years, these stars slowly collapse until the temperature and pressure at their core is sufficient to ignite stellar fusion. At this point, the star is converting hydrogen into helium at it’s core, and has become a main sequence star.
A star like our Sun will remain a main sequence star for about 12 billion years, as long as the fuel lasts in its core.
We have written many articles about stars here on Universe Today. Here’s an article about about two telescopes acting together to image a T Tauri star, and here’s an article about a T Tauri binary ejected from its system.
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There are hundreds of billions of stars in the Milky Way alone; young and old, large and small, quiet and violent. But they all started out in the same way. Let’s take a look at thebirth of stars.
When we look out in the Milky Way, what we see are the stars, but a large part of the galaxy’s mass comes from clouds of molecular hydrogen; the stuff of future stars. These clouds are happy to drift along in the Milky Way for millions and even billions of years until some kind of event causes the cloud to collapse. It could be the collision between two clouds, or the shockwave of a passing supernova. This pushes the cloud over the top and gives gravity a chance to take over, and begin collapsing the cloud together.
As the cloud collapses, big pieces shear off. Each of these will become a star of their own. The mutual gravity on each chunk of the cloud continues to pull the material inward. The conservation of momentum from all the individual particles in the cloud makes it start to spin.
The first stage in the birth of a star is called a protostar. This is where the majority of the stellar material has collected together in ball in the center, but there is a huge disk of gas and dust obscuring it from our view. As long as there is still inflowing material, the object is a protostar. After enough material falls in on the star, jets of material blast out from either pole, announcing the new protostar to the Universe. The protostar stage takes about 100,000 years to complete.
Once there’s no more material falling inward, all that’s left is a hot ball of gas. Astronomers call this stage a T Tauri star. It doesn’t have internal temperature and pressure to begin nuclear fusion at its center, but it’s still a very hot object, and can appear as bright as a regular star. Over the next 100 million years, gravity continues to collapse the T Tauri star until the temperature at its core reaches the point that nuclear fusion can begin.
At this point, the star makes a transition to the main sequence stage of its life. This is a place it’ll remain for millions, billions, and even trillions of years depending on its mass.
