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This image captures about 90×90 degrees of sky, looking south towards the center of our galaxy. There’s a lot to explore and discover here, including spectacular examples of most of the different types of deep-space objects.
The two bright “stars” in the lower part of the image, just left of the Milky Way, are not stars at all, but are the two largest planets in our solar system, Jupiter and Saturn. Each of these planets is 100s of times larger than the Earth; in fact Jupiter is 2.5 times more massive than all the other planets in our solar system combined.
Being easily visible to the naked eye, we have known about Jupiter since antiquity, but we only really started learning when the Pioneer space probes visited Jupiter on fly-by missions in 1973 and 1974, followed by Voyager I and II in 1979, Ulysses in 1992, Cassini in 2000 and New Horizons in 2007. These were followed up by orbiter missions Galileo, from 1995-2003 and currently the Juno orbiter, which arrived in 2016.
Jupiter is huge, roughly 11x the diameter of Earth, which means that it’s volume is over 1300x Earth, but it’s mostly gas; hydrogen 89% and helium 10% by volume but likely with a solid, rocky core 10s of 1000s of kilometers beneath the surface. Jupiter has four large Galilean moons, first discovered by Galileo Galilei in the early 1600s, and each of them is a remarkable world in itself. The closest, Io, has 100s of active volcanoes and is the most geologically active body in the solar system. Europa has a fairly smooth icy surface, with possibly a liquid water ocean beneath the ice. Ganymede is the largest moon in the solar system, slightly bigger than the planet Mercury, and might have a liquid water ocean below it’s icy surface. The fourth moon, Callisto, is composed of roughly equal parts rock and ice, and features the largest impact crater in the solar system, over 3000km wide.
Saturn was also visited by the Pioneer and Voyager space probes, several years after these probes visited Jupiter since Saturn, at roughly 1430 million kilometers from the Sun, is almost twice as far away as Jupiter. In 2004 the Cassini-Huygens spacecraft arrived at Saturn after a 7-year trip from Earth that included passes of Venus (twice), Earth (again), and Jupiter. It then went into orbit around Saturn and spent 13 years studying the famous rings, atmosphere, magnetic field and moons of the giant planet.
Saturn has one very large moon, Titan, that is a remarkable alien world on it’s own, with a thick atmosphere of nitrogen, methane and ethane, and liquid hydrocarbon lakes on the surface. A few months after arriving at Saturn, the Huygens probe separated from Cassini and coasted through space for 22 days before landing on the surface Titan.
At about 1.5 billion kilometers away, it takes light, or radio waves, about 80 minutes to reach Saturn. That’s a long way away, but everything else in this image is 10s to 1000s of light-years away…
Under clear dark skies like we had on this magical night, the Milky Way appears as a mottled, hazy band, with bright star patches obscured by darker bands that look somewhat like clouds. These dark bands are vast clouds of dust and gas that are thick enough to block the light of the millions of stars behind them. These clouds are mostly hydrogen and helium, gas, but also contain trace amounts of all the other elements, including oxygen, carbon, silicon, and iron. All of these heavy elements are created by nuclear fusion in the depths of stars, and then released out into space when the stars explode and die.
In these photos, these clouds appear to be quite dense, but in fact are only about 1/1000 as dense as the best vacuum we can create here on Earth. However, they do contain many 1000s of times the number of molecules as empty interstellar space.
|Molecules / cm3|
|Atmosphere at sea level||50,000,000,000,000,000,000|
|Ultra high vacuum, atmosphere at 250km||1,000,000,000|
|Dense galactic cloud||10,000 – 1,000,000|
Emission nebula appear as glowing red gas clouds in long exposure astro-photos. Why red? What’s going on here is no less amazing than the birth of new stars, formed by the gravitational collapse of massive clouds of gas. The biggest of the new stars are so hot the appear blue and are throwing out a lot of UV radiation. This radiation ionizes the nearly hydrogen gas, and it fluoresces red. This is known as hydrogen alpha emission.
The bright blue of the gas clouds in the M20 nebula is not caused by fluorescence, but by the reflected light of nearby stars. A star’s color is determined by it’s surface temperature, which depends on its size (and age), and these clouds appear blue because the brightest stars in a young star cluster are giant stars that burn blue-hot.
These nebulae got their completely inaccurate names because through small telescopes they appear as small blue/green disks, looking like planets. Through much bigger telescopes they are revealed to be the exploded shells of dying stars. In this image are the planetary nebulae M57 Ring nebula in Lyra, the M27 Dumbbell nebula in Sagitta, as well as smaller, fainter NGC 7009 and 6818.
These nebulae are excellent examples showing how elements produced inside stars are spread out into the galaxy. The M57 Ring nebula is the result of a sun-like star blowing off its outer layers of gas, about 6000-8000 years ago, and collapsing into a white dwarf star. The red is from ionized hydrogen and nitrogen, just inside that is a shell of greenish oxygen, and the blue glow at center is helium. Other elements, determined by spectroscopic studies, include neon, sulfur, argon and chlorine. It may look like a peaceful shell, but in reality it’s a tremendous explosion, with the gas still expanding through space at 20-30 kilometers per second. More information at NASA and SEDS.org.
Globular star clusters
Scattered mostly throughout the constellation Sagittarius are a dozen fuzzy objects marked “globular”. Telescopes reveal that these are dense balls of 100,000s to millions of stars: globular clusters.
These clusters are ancient, containing some of the oldest stars in the galaxy, with ages between 11 to 13 billion years, nearly as old as the universe itself. There are just over 150 of them around the Milky Way, most orbiting fairly close to the center of our galaxy, but swinging above and below the disk of our spiral galaxy where most of the stars reside. Most other galaxies are also surrounded by swarms of globular clusters, for example over 300 globular clusters have been cataloged in our nearest neighbor, the M31 Andromeda galaxy.
The stars in these clusters are packed 1000s of times closer than the stars around our sun. This is so close together that any planetary systems around stars in globular clusters would not likely survive because encounters with nearby stars destabilize planetary orbits.
Hunt around in the image, see if you can find all these Messier globular clusters: M2, M9, M10, M12, M14, M15, M22, M30, M54, M55, M56, M69, M70, M71, M72, M75, M92, M107.
Directly above Saturn in the image is a faint, fuzzy patch labelled NGC 6822. Barnard’s galaxy is a relatively nearby dwarf galaxy, and one of the members of our Local Group of galaxies. Even the smallest galaxy is still a immense structure, and this one is about 7000 light-years wide and contains about 10 million stars; only a dwarf when compared to big galaxies like our Milky Way, with about 400 billion stars.
Stars beyond counting, giant planets, star clusters young and old, stars dying and being born, massive gas clouds both cold and hot, 24 of the 110 deep-space objects on Charles Messier’s famous 18th century catalog, and the center of our galaxy of over 100 billion stars. Plus mountains in all directions here on Earth.
Not a bad view from this little ridge top.