This wide view mosaic image captures about 70×80 degrees of the Northern Milky Way. Zoom in and explore this rich part of our galaxy.

Exploring the autumn Milky Way

There’s a lot to explore here!

When exploring on Earth, it makes sense to use metres to measure distance; 1/1000th of a meter for little things, or 1000s of meters for big things. But space is so incomprehensibly vast that kilometres are just not big enough. The Sun is about 150,000,000 kilometres away, a number that is bigger than we can imagine. The farthest apart any two places on Earth can be is 20,000km, and the sun is 7500 times that distance away from us.

Another way of measuring distance is with time – how long does it take to go there? For example, the ancient league unit originally meant about as far as a human could walk in an hour. It turns out that the speed of light, the fastest thing there is at 299,792 km/s, is a convenient way to measure the vast distances in space. To get a feel for how fast that is, light goes around the Earth 7.5 times in one second, and it takes sunlight over 8 minutes to reach us from the Sun.

The next closest star to Earth (Proxima Centauri) is over 40,000,000,000,000 kilometres away – that’s 40 million million kilometres – and that number is so large as to be meaningless. In order to bring the numbers down to a scale that we can understand, distances to stars are often measured in light-years, which is the distance that light travels in one year. Doing the math, one light-year is around 9.5 trillion (9.5 million million) kilometres. By that measure, the nearest star to the sun is “only” 4.3 light-years away.

Actually… distances to stars are measured in parsecs, which is the only practical way for astronomers to directly measure such extreme distances. The distance in parsecs is then translated into lightyears.

Now that we have our tape measure, let’s start exploring…

The Milky Way

Our home in the Universe, the Milky Way galaxy, is a vast disk of stars and gas. We are inside this disk, about 1/2 way from the centre. On a clear night the Milky Way appears as a hazy band in the night sky because we are looking through the disk, and there are so many stars that they blend in to a hazy cloud. The hazy swath of the Milky Way is also mottled with dark patches, which are vast clouds of interstellar gas and dust, thick enough to block the light of the stars behind them.

These interstellar clouds may appear dark and dense, but are actually very close to a perfect vacuum, and only block the light of distant stars because they are so vast; 100s of light-years thick.

Also visible in the zoomable image are patches of red, which are glowing clouds made of the same gas / dust mixture as the dark clouds. They are glowing because extremely bright, nearby stars are putting out enough energy – especially UV light – that the hydrogen gas in the clouds is ionized. In these hydrogen emission regions, things get really interesting, because in most cases, the bright stars are newly-formed from the gravitational collapse of the clouds themselves.

Within these red clouds, we are witnessing the birth of new stars and solar systems.

Heart and Soul nebulae

Between the constellations of Perseus, Camelopardalis, and Cassiopeia are two vast star forming regions called the Heart and Soul nebulae. If they were visible to the naked eye, these nebulae would appear almost 12x the size of the full moon, and are around 600 light-years wide. Looking at the diagram of the Milky Way, there are two major arms that spiral outwards from the centre. We are located in the minor Orion Spur arm – a bridge of stars between the two major arms – and the Perseus arm is the next major spiral arm outwards. These nebulae are in that Perseus arm, 6000 to 7000 light-years away, and are two parts of the same vast cloud of gas.

These gas clouds are collapsing and forming new stars of all sizes, including massive stars that are 1000s of times brighter than our sun that shine blue/white, radiating intense UV light. That UV light has enough energy to ionize the surrounding gas cloud, causing it to glow red and creating a hydrogen emission nebula.

The young star clusters at the heard of these nebulae are spewing out torrential stellar winds that are carving out massive bubbles in the surrounding gas. This process is quite evident in these excellent images by Trevor Jones. These images were acquired with narrowband optical filters which increase the signal-to-noise ratio, and Trevor gathered a lot of data: 6 hours of imaging time compared to my 16 minutes.

At the heart of the Heart nebula is the Melotte 15 cluster, only 1.5 million years old, and actively eroding the surrounding dust and gas with the stellar wind blowing away from these brilliant young stars. This erosion can be clearly seen in this beautiful close-up image by Quebec astrophotographer Karl Theberge.

Astrophotography is all about two things: gathering lots of image data, and then processing it into an image. This excellent image of the Heart and Soul nebulae by Terry Hancock shows what’s possible with over 56 hours of exposure time gathered over numerous nights over three months.

Perseus double cluster

Just to the right of the Heart and Soul nebula is the stunning Perseus double cluster. These twin clusters, cataloged as NGC 869 and NGC 884, are easily visible to the naked eye under a dark sky, and have been known since antiquity. They are over 7000 light-years away and appear related to each other: they are a few hundred light-years apart and their stars are of a similar age, about 14 million years. This is strong evidence that these clusters formed together, from the same star-forming region – the same process that is currently occurring in the nearby Heart and Soul nebula.

Each of these clusters contains at least 300 brilliant blue/white giant stars, each of which emits 1000s of times more light than our sun.

Visible in this beautiful image are seven brilliant orange stars, called red supergiants. These stars are near the end of their lives, have fused most of their hydrogen into helium, and have swelled up to 100s of times their original size.

At this late point in their lives they fuse helium for a couple million years, then carbon for only a few thousand years. What happens next depends on the mass of the star, but if the star is massive enough, it starts to ever-more rapidly produce the heavier elements until the core becomes iron. At that point, the star collapses and explodes as a supernova, filling space with the heavy elements forged in their cores.

These brilliant stars live short, intense lives that end spectacularly, seeding the galaxy with the heavy elements that later form planets… and life!

California nebula

Just above the left foot of Perseus is a broad smear of red that is the California Nebula, NGC1499.

Emission nebulae are regions of otherwise dark, cold gas that glow red because the gas has been ionized by the energy emitted from nearby, brilliant stars. Often, those stars were newly created from the collapse of the gas cloud itself, but not in this case.

Here, the bright red glow of the California nebula is caused by the torrent of energy pouring out of the single blue giant star Menkib (or Xi Persei), the bright star immediately to the right of the nebula. Menkib is about 30x the mass of our Sun, over 12,000x brighter (in visible light) and puts out 263,000 times the power of our Sun (including ultraviolet and other wavelengths invisible to the eye).

This immense nebula would appear 6x the width of the full moon, if it was visible to the naked eye. It appears three degrees wide in the sky, which, at 1500 light-years distance means this glowing cloud is around 75 light-years long – absolutely massive – and is being illuminated by a single star.

My four minutes of exposure time really doesn’t do this nebula justice – this beautiful false-colour image by Yannick Akar is much deeper and more detailed, incorporating many hours of image exposure time. Here’s another excellent California Nebula image by Terry Hancock.

Flaming Star nebula, IC405

This unique emission nebula is in the constellation Auriga, just above the light-painted hoodoo in the zoomable image at the top of the page, but a little washed-out by the sky glow close to the horizon.

Similar to the California Nebula, this nebula is also glowing due to the energy output of a single star, AE Aurigae. This brilliant star both ionizes the gas and illuminates the nebula, causing it to glow red from hydrogen ionization and blue from the reflected light of the star.

But this is no ordinary star – this massive blue star is a runaway star that is sailing through the galaxy at around 100 km/second, relative to us.

The relative to us part is important, because movement only exists relative to something else. For example, how fast are you moving, right now? If you’re reading this from your couch or at your desk, you might answer “I haven’t moved since Covid hit!”. If you’re on a train or in a car, you might answer “about 100 km/h”. But those are both relative to the surface of the Earth. But the Earth is spinning – once around ever day – so your actual speed, relative to the centre of the Earth, is 1676 km/h if you live on the equator. The Earth is also orbiting around the Sun, which means relative to the Sun, you’re moving at about 30 kilometres per second, just sitting there on your couch.

But it doesn’t stop there, because the Sun is orbiting the Milky Way, and the Milky Way is flying through the Universe. Feeling dizzy yet? Here’s a fantastic article that dives deep into the question of just how fast are you moving?

As stars orbit the centre of the galaxy, they also move relative to each other, orbiting the centre of mass of a star cluster for example. A close encounter with another star, often within its birth cluster, can slingshot a star out of a cluster and send it careening across the galaxy at high speed.

AE Aurigae is one of the these runaway stars, and by tracing it’s observed motion backward, it appears to have originated in the Orion Nebula around 2 million years ago. Two other stars were also ejected from the nebula (53 Arietis and Mu Columbae), and all three stars are moving away from each other at 200 km/second.

Here’s a fantastic image by Pavle Gartner, much wider field, showing IC405 and its nebula neighbor, NGC1893.

Pacman nebula, NGC 281

The beautiful Pacman emission nebula is in the constellation Cassiopeia and appears quite small only because it’s very far away, at about 9200 light years. But actually, it is a vast region nearly 100 light-years across – because everything is space is vast and very far away!

Like most emission nebulae, Pacman is actively forming new stars, and at it’s centre is a cluster of five blue supergiant stars, each one putting out 1000s of the times the energy of our Sun. That torrent of energy is evacuating an immense cavity in the surrounding gas cloud, and has eroded the gas into towering columns – those columns alone are several light-years long.

Why are some of these nebulae images mostly red and others mostly orange? The red toned images are close to the colours you would see with your unaided eye, while the orange-toned images use an artificial colour scheme that maps different ionized elements (hydrogen, sulfur, oxygen) to green, red and blue, which reveals more structural details in the nebulae. Trevor Jones has a great article on narrowband imaging that explains how this works.

This detailed image reveals the interior dust columns beautifully. And another great image here at AstroDon imaging.

NGC 7822

Just to the left of Cassiopeia is the glowing red NGC 7822 nebula complex.

This massive star-forming region lies about 3000 light-years away in the constellation Cepheus. It is also actively producing stars, and contains one of the brightest (relatively) nearby stars to Earth, a massive blue-white giant star that is 100,000 times brighter than our Sun.

The nebula also contains a supernova remnant from an earlier massive star that exploded after only a few million intense years. On cosmic time-scales, these nebulae don’t live for long – only a few million years – because their gas is blown away by the stars they create.

Clearly visible in this spectacular false-colour photo by Mark Carter are pillars of primordial gas and dust that are being eroded by the torrent of energy put out by the young stars at the centre of the nebula. These pillars are several light-years long – with one light-year being about 9500 billion kilometres – and often contain stars that are still forming.

Andromeda and Triangulum galaxies

Captured in the upper right portion of the Dinosaur park image are our two nearest big galactic neighbours – the Andromeda galaxy, M31 and the Triangulum galaxy, M33.

At around 200,000 light-years in diameter, Andromeda is the largest in our local group of galaxies, our Milky Way is next at between 100,000-200,000 light-years, and the Triangulum is third at about 50,000 light-years in diameter.

Far larger telescopes, much more imaging time, and more complex data processing can image the bright star-forming regions in other galaxies. These bright red clouds are clearly seen in these images of the Andromeda and Triangulum galaxies. Both galaxies are very rich with these star forming regions, as well as dense clusters of young, brilliant blue stars which have blown the dust away from their birth clouds.

Almost all of the stars and nebulae we see in the Milky Way are within around 10,000 light-years, but these two galaxies are much, much farther away. Andromeda is about 2.5 million light-years away, and Triangulum slightly further at 2.7 million light-years, and both are (barely) visible to the naked eye under very dark skies, making them the farthest objects you can see with unaided eyes.

Andromeda is relatively easy to see, and was first cataloged around the year 964 by the Persian astronomer Abd al-Rahman al-Sufi who described it as a “small cloud”. It was not until 1745, at least 150 years after the invention of the telescope, that the true nature of this “small cloud” was glimpsed by the French mathematician Pierre Maupertuis, who suggested that Andromeda may be a much more distant “island universe” than all the other nebulous clouds visible in the night sky.

The true (incredible) nature of this little cloud of light in the northern sky wasn’t settled for another 150 years, during which time is was the subject of the great debate in astronomy: do these “spiral nebulae” lie within our galaxy, or are they independent galaxies that are much further away than anything else in the sky? The debate was not settled until 1925 when Edwin Hubble (the namesake of the famous space telescope) was able to estimate the distance to Andromeda.

Because these are the two closest galaxies to us, they are the most studied and photographed, and the internet is full of incredible images, some representing 1000s of hours of work and billions of dollars in equipment. This incredible composite image of Andromeda was assembled from Hubble, Subaru, and other professional telescope data sources. And this stunning image of Triangulum was likewise assembled from data acquired by the Hubble and Subaru telescopes.

Everything is part of something bigger, and so our local group is part of the much larger Virgo Supercluster, which contains at least 100 other galaxy clusters, and is part of a filament of galaxy clusters stretching for 100s of millions of light-years across the overwhelmingly vast universe.

More…

The Pleiades are a favourite winter star cluster, easily visible to the naked eye. This winter I was pointing out Orion to a friend’s 10-year son when he excitedly pointed up to the Pleiades and asked “what’s that?”. I explained that is a cluster of new-born stars, and it’s also what your parents car is named after – the Pleiades are known as Subaru in Japanese. The stars are around 100 million years old – still “young” for stars, and at a distance of 444 light years it is one of the nearest clusters to Earth.

At the foot of Perseus lies a filament of dust and gas spanning the distance from the star Atik to the NGC 1333 nebula, beautifully captured in this fantastic deep image. NGC 1333 itself is an incredible stellar nursery that contains 100s of stars that are still forming, mostly hidden from view by thick clouds of dust. Its dark clouds are subtly lit from within by glowing gas that has been shocked by jets of matter emitted from the still-forming stars. It is also glowing faintly blue from the reflected light of an extremely large blue star, one of the first-born in this cluster.

And just rising above the hoodoos is the very first object in Charles Messier’s 18th century catalog – the M1 Crab nebula – which is the remnant of a star that exploded in the year 1054, bright enough to see in the daytime, and was recorded as a “guest star” by astronomers in China, Japan and Persia.

The night sky is full of wonders – all you need to do is look!

– Darren Foltinek, November 2021