My friend Rob and I spent May in the US south-west desert area of Utah, Arizona, and Nevada for a month of biking, climbing and canoeing. In the middle of the month we found ourselves camped on Gooseberry Mesa, just south-west of Zion National Park and east of St. George. Camping on the mesa is wild, with no water, power or food, but there are fantastic bike trails, well-maintained outhouse facilities, stunning views of the big peaks of Zion National Park, and the camping is free.
After the days ride and our camp dinner I retire to the tent. There are some high cirrus clouds in the sky, and the alarm is set for 2am with the hope that the clouds dissipate and can I do some night photography.
In order to view these dark-sky images properly, please make sure that your screen is bright enough. Adjusting the Contrast and Brightness of your monitor should allow you just barely distinguish between the different shades of gray on the following strip.
Wide field view
After a few short hours of sleep the alarm goes off at 2 and I have an inner battle between staying in the warm, cozy sleeping bag and getting dressed, going outside and looking at the stars. The inner battle goes on a for a few minutes as I roll around in the bag, procrastinating.
My first thought on poking my head outside the tent is “well, the stars are certainly out, but those cirrus clouds are still there”. As I slowly gain consciousness, reality reveals itself – those are not clouds – that hazy band of bright and dark patches arching across the sky is the Milky Way! Whoa!
Suddenly much more awake and excited, I assemble the camera gear, align the star-tracker to north, manually focus the 28mm lens and start shooting a wide-field panorama.
The star-tracker follows the rotation of the Earth and allows for exposures of up to three minutes or so. This image is a composite of 30 frames, each a two-minute exposure, and covers nearly 180 degrees from south (right) to north (left). Capturing all these frames took about an hour and a half, and now it’s 3:30, the Milky Way is high in the sky, there is still no moon and the starlight is bright enough to walk around the desert without a headlamp. An absolutely beautiful night!
The Milky Way has been known to humanity since ancient times, but its nature was not understood until 1609, when Galileo Galilei first turned his newly-invented telescope towards the Milky Way and discovered that its glow came from individual stars “so numerous as almost to surpass belief”.
There are three things that really stand out when looking at the above wide-angle view of the Milky Way and comparing the northern end (left, towards Cygnus) to the southern end (on the right, towards Sagittarius).
1) The southern part is much brighter than the north. This is because, when looking south towards Sagittarius, we are looking towards the centre of the Milky Way, and there are vastly more stars in that direction.
2) The southern part has more prominent dark clouds than the north. The dark clouds are visible because they are silhouetted against the bright background of billions of stars. Because there are more stars looking towards the south, the clouds appear more prominent.
3) The southern part appears more yellow, while the north is more blue. Looking towards the south we are seeing the galactic bulge, which is a dense collection of mostly older stars near the centre of the galaxy. Most stars in a spiral galaxy orbit the centre in a relatively thin, flat disk, however, the bulge stars orbit the centre of the galaxy in a roughly spherical shell, both in the plane of the disk as well as perpendicular to it, and all angles in between.
Under dark skies, the Milky Way appears as a hazy band of light and dark areas stretching across the sky. The light areas are thick clouds of stars, and the dark areas are clouds of interstellar gas that block the light of the stars behind them. These dark clouds are easily visible to the naked eye, but a long camera exposure really brings them out.
Like everything in the Universe, these dark clouds come in a wide variety of shapes and sizes. The large clouds are obvious, while the small ones are barely visible in the 50mm frames, for example star cluster NGC 6520 and it’s dark-cloud neighbour Barnard 86.
These dark clouds may look thick and dense, and obviously are dense enough to block the light of the stars behind them, but compared to our atmosphere they are nearly a perfect vacuum. However, they have 10,000 – 1,000,000 times as many molecules as normal interstellar space, which contains on average only one single molecule per cubic centimetre.
These clouds contain mostly hydrogen, but also a mixture of all the other elements, and even complex organic molecules. Gravity and pressure can cause these clouds to collapse, giving birth to stars and solar systems.
|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|
We can’t see what these dark clouds in our galaxy look like from the outside, but we can certainly see the dark clouds of dust and gas that swirl in other galaxies.
By measuring the distance to the dust clouds and bright nebula within the Milky Way, astronomers are able to produce maps of our galaxy that define its structure and shape.
NGC 6744 is a galaxy about 30 million light-years away, in the southern constellation of Pavo, that has very similar structure to the Milky Way, but is twice the size.
By 3:30 I’ve finished shooting the wide-field image. The sky rotates 30 degrees every 2 hours, and Milky Way is now standing nearly vertically in the southern sky. There is no moon or clouds and the night sky is still incredibly gorgeous. Sleep can wait until tomorrow afternoon, so I put the 50mm lens on the camera to capture the southern part of the Milky Way in more detail.
It takes another two hours while I slowly scan the camera across the southern sky, capturing 28 more frames. The shutter opens up for two minutes at a time, recording a few photons of light that have been traveling for many 1000s of years to finally arrive here in the desert of Utah and land, plonk, plonk, onto the tiny sensor in the camera.
Click to zoom into the high-resolution image.
What can you find in this image? This part of the night sky is rich with star clusters, globular clusters, nebulae, and of course, so many stars they look like fog. Saturn and Mars are bright and easy to find. How about the center of the galaxy? If you’re keen, try and find all 47 labeled globular clusters.
When looking at the constellation Sagittarius we are looking toward the galactic centre, the very core of our galaxy. This part of the night sky is incredibly rich with star clusters, glowing nebulae, dark gas clouds, and star fields so dense they appear like thick fog.
Unfortunately, for someone living in western Canada, the centre of the Milky Way only rises above the horizon in summer, and never much over 10 degrees, which leaves it in the thick soup of the atmosphere. In the desert of Utah, however, it rises much higher, and camping under these rich skies is a real treat.
At the top-right of this image is M24, the Sagittarius Star Cloud. At first, it appears at first be an enormous cluster of stars, spread out in a broad swath. It is not actually a cluster, but a clear window in a region thick with dark gas clouds. These dark clouds of galactic dust and gas are thick enough to block the light of the stars behind them, but the unique M24 window reveals the rich star fields otherwise hidden behind the clouds.
Fun fact: nebula means “mist” in Latin, and Sagittarius is home to some spectacular and complex clouds of mist.
Messier 20, the Trifid Nebula, is visible at upper-right in the above image, and is part of an enormous gas cloud that is collapsing and forming new stars. M20 is unique in that it combines three characteristic types of nebula in one. Glowing red gas surrounds a tight cluster of hot, bright and very young stars that are putting out ultra-violet (UV) radiation. The UV radiation from these hot young stars is intense enough to ionize hydrogen in the gas cloud, causing it to glow red. This is a hydrogen emission nebula.
The blue gas at the top of the nebula is reflected light from other, very bright young stars. These stars are emitting more blue light, but not enough UV to ionize the gas surrounding them. This is called a reflection nebula.
In front of these bright regions are bands of dark gas, silhouetted against the red and blue glow. Not surprisingly, these are called dark nebula.
Both the Trifid and Lagoon nebula have active star-forming regions at their cores, and the formation of stars and planetary systems is a very active area of astronomy research. The biggest telescopes in the world have revealed remarkable details within these nebulae, including pillars of gas being eroded by the radiation of young, bright stars, jets of gas being ejected during star formation, and even young stars surrounded by a debris disk that will likely form planets – a new solar system in the making.
The stunning image at right, assembled by Robert Gendler from data collected by the Subaru telescope, the Hubble Space Telescope, and Martin Pugh, shows examples of these star-formation processes. More information can be found at this Hubble news release page and this excellent discussion of M20.
Bright nebula are the birthplace of stars and planets, and star clusters are what results after the stars have ignited and blown the original gas cloud away.
Star clusters are just what their name suggests: a group of stars of similar age, roughly bound together by gravity. All the stars in a cluster were created by the same collapsing cloud of gas, and are roughly the same age.
Messier 7, visible at the bottom-right of the Sagittarius image, is one of the brightest clusters in the sky, easily visible to the naked eye and has been known since antiquity. Star clusters typically contain between 100 to 1000s of stars.
The biggest brightest stars in a cluster live very short lives, fusing hydrogen into helium, then helium into carbon and oxygen, followed by nuclear reactions that produce neon, magnesium, silicon, and heavier elements all the way to iron. After only a few 10s of millions of years, these massive stars explode into supernova, scattering the heavy elements that were forged in their innards out into space, where they can form the next generation of stars.
This process fills the galaxy with all the elements that later form planets and make life possible.
You and I and everything we know are literally made of star dust.
Following the laws of black-body radiation, just like an old-fashioned incandescent light bulb, a stars colour is directly related to its temperature, with red on the cooler side and blue on the hotter side.
The relationship between a star’s mass, lifetime, luminosity and surface temperature (colour) was first characterized around 1910 and summarized in the Hertzsprung-Russell diagram shown at right.
Massive, hot stars burn though their fuel extremely quickly, and only survive a few 10s of millions of years before exploding. Because star clusters contain relatively more of these young, massive, blue stars, the clusters usually appear blue compared to older, smaller and cooler background stars.
The Sun was once part of a star cluster, but because it is quite old, about 4.6 billion years, the cluster that the Sun was born in has dissipated and drifted apart.
Simulation of millions of years in the life of a star cluster. Credit: F.I. Pelupessy.
This simulation shows how star clusters fling their smaller stars out of the cluster through gravitational interaction, spreading these stars throughout the galaxy.
Scorpius is a large and bright constellation just south of the centre of the Milky Way. It contains a large number of deep-space objects, including 14 globular clusters, at least 10 star clusters, and several emission, reflection and dark nebula. Also in May 2016 the planet Mars happened to be passing through the head of Scorpius, dominating the constellation’s stars with its bright red glow.
The stinger of Scorpius is embedded in the middle of the Milky Way, while at the head lies a particularly dramatic complex of dark gas clouds and glowing nebula called the Rho Ophiuchi complex.
The Rho Ophiuchi complex is one of the most colourful nebula, and includes regions with red hydrogen emission, blue and yellow reflection, and silhouetted dark foreground gas clouds.
The brightest star in the cloud complex is Antares, a red supergiant star that is living a short, furious life, throwing off it’s outer layers and forming the yellow nebula around it. One of the largest stars visible to the naked eye, it is almost 900 times the size of our Sun, and is destined to explode as a supernova in less than one million years.
The stunning image of Rho Ophiuchi is part of an extremely high-resolution mosaic of the centre region of the Milky Way. Totalling 1 billion pixels, it was put together as part of the European Southern Observatory GigaGalaxy Zoom project as part of the International Year of Astronomy in 2009.
Also in this region are three globular clusters, M4, M80 and NGC 6144, which are all much bigger but farther away than the glowing gas clouds making up the nebula.
What’s the difference between a star cluster and a globular cluster?
Age: star clusters are quite young, typically 10s of millions of years, while globular clusters are billions of years old.
Size: star clusters contain from 100 to several 1000 stars, while globular clusters contain 10000s to millions of stars.
Location: star clusters are found inside the flat disk of our galaxy, as well as other galaxies, while globular clusters are found in wide-ranging orbits that swing far from the disk of spiral galaxies, like ours, and have been observed surrounding most other galaxies.
Shape: because globular clusters are so large, and mostly outside of the galaxies disk, they are able to maintain a roughly spherical shape, whereas star clusters have irregular shapes.
Messier 4, in the middle of the Rho Ophiuchi complex, is one of the closest and best-studied globular clusters in our galaxy. Observing M4 in 1764, Charles Messier was the first to be able to resolve individual stars in a globular cluster, and modern studies performed by the Hubble telescope have revealed ancient white dwarf stars that are approximately 13 billion years old, among the oldest known stars.
How globular clusters form is still a mystery. They are truly ancient, up to 13 billion years old, which makes them only slightly younger than the age of the Universe, 13.8 billion years. Are they ancient, “baby” galaxies, that formed in regions of space that never had enough gas to create a big galaxy? Maybe. Are they the left-over cores of dwarf galaxies that were absorbed by larger galaxies? Maybe.
There are around 150 – 160 globular clusters in orbit around our Milky Way, and the zoomable “detailed view” image at the top of this blog shows dozens of visible globular clusters, 48 to be exact. These globulars are grouped around the galactic centre, in the constellations Sagittarius, Scorpius and Ophiuchus, and this clumped distribution of globular clusters in the sky was used by Harlow Shapley, in 1918, to make the first estimates of the dimensions of the Milky Way, and determine the approximate location of our Sun within the Milky Way.
Globular clusters are both very old and relatively dense with stars. Where we are, in the ‘suburbs’ of the galaxy, the nearest star to our Sun is 4.5 light-years away. In a globular cluster, the stars can be on average one light-year apart, and in the core of the cluster, can be as close together the size of our solar system.
Where is the centre of our Milky Way is a question that has been puzzling astronomers for 100s of years.
And shortly after that question came the 2nd question: What is at the center of our galaxy?
The thick dust and gas clouds obscure our view of almost 20% of the sky, including the very centre of our galaxy, in visual wavelengths of light. It was not until the development of radio astronomy in the 1940s and 50s that astronomers were able to see through the dust. This led to the discovery of an intense point-source of radio emissions in Sagittarius, and the location of the centre of the Milky was determined.
With the development of powerful telescopes that can observe in infrared and radio (longer wavelengths), astronomers were able to clearly observe the very centre of our galaxy. And what they found were stars, orbiting extremely rapidly around an invisible point.
The speed that a small object orbits a much bigger one (like a satellite around the Earth, or the Earth around the Sun) depends on only two things: (1) the mass of the larger object and (2) the distance from the larger object. Therefore, by observing the orbit, the mass of the large object can be determined.
Between 1995 and 2013 the movement of stars around the galactic centre was observed using the massive 10m diameter W. M. Keck telescopes in Hawaii. An analysis by the UCLA Galactic Center Group using Keplers laws of orbital mechanics to the observed orbits of these stars shows that an invisible object with a mass of 4 million (!) Suns sits at the very centre of our galaxy – a super massive black hole.
So what does the black hole at the centre of the Milky Way look like?
Usually nothing. It is invisible, until it swallows some gas, or a star strays too close. Then the black hole erupts into life, producing some of the most violent and spectacular displays in the cosmos as the inflowing gas spirals around the black hole at massive speeds, heats up to stellar temperatures, and spews intense radiation.
A burst of x-ray radiation was been observed by the Chandra X-ray telescope in September, 2013.
Most other galaxies are thought to harbour supermassive black holes at their centres. When these black holes are feeding on matter, the galaxy is said to have an active galactic nucleus, spewing jets of matter far out into inter-galactic space. By combining visible images with radio and x-ray data, the jets of gas spewing from active galaxies have been spectacularly imaged.
It’s probably best for us that we have a mostly quiet, dormant black hole at the centre of our galaxy…
To clear, dark skies full of beauty and wonder!
– Darren Foltinek, 2016