In the first three posts of this series (Part 1 – Origins, Part 2 – The Formation of Stars, and Part 3 – Planets and the Conditions Necessary for Life), we discussed our origins from the Big Bang to the formation of our solar system. and the basic ingredients that allowed life to develop and flourish on our planet.
In this latest installment, we look at what may happen next. As in the first three articles, we will use images taken from our observatories in Killarney Provincial Park.
All the good things…
Stars find themselves in an epic battle between two forces: the crush of gravity and the burst of radiation from nuclear fusion within their cores.
This enormous tug-of-war plays out over the life of the star. However, as stars begin to exhaust their main fuel source, hydrogen, they must switch to other sources to generate thermonuclear heat. In the end, stars will die in different ways depending on their mass when they first formed.
As we learned in our second installment, stars are created from hot balls of gas that have condensed from nebulae.
However, not all of these condensations have the same mass. Some condensations formed much more massive stars than others. Today, astronomers have seen stars from as small as 0.1 times the mass of our Sun (OGLE-TR-122b) to several hundred times more massive (R136a1), with nearly 90% of all stars equal to or less mass than our own Sun. Of the remaining 10%, few exceed 20 solar masses.
Mass and the destiny of a star.
Most stars spend their lives according to what we call the “main sequence.” This is a way of classifying a star based on its normal conversion of hydrogen to helium.
More massive stars require more heat to counteract their tremendous gravities. They live long and die quickly, consuming available fuel in just a few tens of millions of years. However, their surface temperatures are very high because their cores produce a lot of heat.
Stars smaller than our Sun can live billions or even trillions of years. These stars have relatively little mass to crush them and do not require as much energy output to recoil, which keeps their surface temperatures cooler.
Astronomers note that the higher the star’s surface temperature, the bluer it appears. Likewise, the colder its surface temperature, the redder it appears. A plot of surface temperature versus stellar mass is what largely defines the “main sequence.”
So far, we have described the basics of stars that convert hydrogen to helium and live their lives according to their mass. But what happens towards the end of their lives depends on their initial mass.
The last chapter in the life of a star.
The end of the life of a Sun-like star is fascinating and complex, far beyond the scope of this discussion.
When a star like our Sun runs out of hydrogen in its core, it begins to contract and heat up from the inside. Its outer layer of gas swells and cools, becoming redder in the process. Astronomers call it the Red Giant Star.
The double star, “Albireo”, photographed by the 0.41 meter telescope in the Kchi Waasa Debaabing dome of the Killarney Provincial Park Observatory Complex.
The image above of the beautiful double star Albireo shows two stars of different colors. Their colors are directly related to their mass and life stage.
The blue star on the left is a hot main sequence star that has more than twice the mass of our Sun. Its surface temperature is very high, creating the blue color. In contrast, the amber star on the right is an example of a star that left the main sequence and is now a red giant star. Its outer atmosphere has swollen and cooled, hence the red color. 
Planetary nebulae: the beautiful agony of stars like the Sun
The vast majority of stars similar to the Sun will shed a large amount of material in their last million years of normal life.
This material receives energy from radiation from a hot central star (more on this star in a moment) and re-emits light that we can easily see.
M57, the Ring Nebula imaged by the 0.41 meter Telescope at the Killarney Provincial Park Observatory Complex
M57, the Ring Nebula (above) is an example of a Planetary Nebula. This type of emission nebula looks like a ring, but is actually spherical in shape. “Planetary” nebulae were so named because in the low-power telescopes of those who first observed them hundreds of years ago, they resembled planets in shape and relative size.
That hot central star is what astronomers call a white dwarf, the hot carbon core of what was once a main sequence star. White dwarfs can continue to shine for billions of years, but eventually they will fade into a black dwarf, the dead ember of a once bright star.
The end of the life of a gigantic star
Antares is a reddish star visible in the southern sky within the constellation of Scorpio the Scorpion. In addition to its beauty, it is one of the largest and most massive stars easily visible in the night sky.
Antares is almost 700 times larger and more than 10 times more massive than the Sun. If you put Antares where the Sun is, the planets Mercury, Venus, Earth and Mars would all be inside the star! It is one of the few stars where we can actually see structure on its surface (see the first image in the video below).
Stars the size of Antares, known as “supergiant stars,” can grow so large because their enormous mass puts great gravitational pressure on their cores. There is so much pressure that each star can go through many cycles of:
- core compression
- swelling and cooling its outer atmosphere, and
- nuclear fusion of existing core material into new elements (see diagram below) .
 Summary based on NASA’s Imagine the Universe table, Large Stars, Table 1
The creation of iron: the beginning of the end
Near the end of a massive star’s life, it will resemble an onion. Various types of nuclear reactions occur at different levels within the star, and the end products of one nuclear reaction become the raw ingredients of the next.
This process continues until the star produces iron. Melting iron requires energy rather than producing it. Once a star produces iron, it has reached the end of its nuclear fusion capacity.
When the iron core can no longer resist the tremendous gravity, the star begins to implode. Without going into more detailed physics, all matter cascades and crushes the atoms in the nucleus. Protons and electrons come together to form neutrons. The neutrons act as an impenetrable wall against which the outer layers of the star collide.
The collision causes the material to bounce outward, colliding with more material that still falls inward. The energy from this collision creates a tremendous explosion that we call a supernova.
Supernova: the astonishing end of a massive star
In less than a quarter of a second, a supernova explosion tears away the outer layers of a star. These layers contained the atomic elements formed during the life of the original star, plus heavier elements formed during the supernova itself.
The following animation begins with an image of the star Antares taken by a team from the European Southern Observatory led by K. Ohnaka. Time has been sped up to show the process of the star collapsing in on itself.
In this imaginary Antares supernova, the colorful layer of material ejected from ancient Antares represents the various layers of fusion products we have already discussed. The final image is of the supernova remnant we call the Veil Nebula. Does that image look familiar to you? It is the same Veil Nebula supernova remnant that we described in our first post in this series.
With this final image, we have closed the circle of our journey. Supernovae leave their leftover materials in the interstellar medium. These materials collide with existing gas clouds, enriching them and starting the star formation process again.
An intergalactic light show
Supernovas are amazing events that can eclipse an entire galaxy for weeks. They release more energy during their 40-day lifespan than the Sun will release in its entire 10 billion year lifespan.
Two stars visible in the northern hemisphere, Antares and Betelgeuse, are good candidates for becoming supernovae.
Below is an image taken before and during a relatively nearby supernova that occurred in 1987. It was first observed by Canadian astronomer Ian Shelton at the University of Toronto’s Las Campanas Observatory.
Image of Supernova 1987a (Ian Shelton supernova). Australian Astronomical Observatory Photo: David Malin
Coming back down to Earth
Think about the last time you gathered around the campfire with friends and family.
You basked in the warmth of the fire, bathed in its light and were mesmerized by its flickering glow. After a wonderful day of hiking, canoeing, and swimming in “your” park, you felt connected to earthly beauty, and yet there was still a mystery about you.
Dante Alighieri wrote in his 14th century work, Divine Comedy“The heavens call you and revolve around you, showing you their eternal beauties, and your eyes still look at the ground.”
With this four-part series, we hope to have given you the opportunity to lift your eyes to the heavens while giving you a greater appreciation for the life and soil below.
The next time you are sitting around a campfire in one of our many provincial parks, take a moment to look up at the sky and think about the connection between the fire in front of you and the “fires” in the sky.
This concludes our series on the astronomical origins of the universe. If you haven’t had a chance to read them, we encourage you to go back and start from the beginning of this series.
 Apart from some images captured with special techniques or the Hubble Space Telescope, we cannot see or photograph stars other than as points of light. In this image by Alberio, the size of the star is merely a photographic artifact and should not be taken as a sample of its actual physical size.