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Today, we are going to take a look at the different colours of galaxies, and some of the reasons behind the different colours. Just a couple of videos ago we took a look at some galaxies and how those galaxies are often classified by shape.

Now that we’re getting into the month of February, our own galaxy is starting to be visible once again in morning time. So we’re going to move forward to just before sunrise, bringing up the Summer Triangle over in the east and Scorpius down here towards the south. If we head out to the countryside, and we are going to head out to a perfectly dark sky this time, Bortle CLass 1, early in the month we’re not really getting to see the glow. You might see a little bit of the Milky Way, but we’re not really getting a good view thanks to the Moon, which will still be very close to full early in the month. On the 4th, the Moon is 93.4% full, still pretty full, definitely producing enough light to interfere with our view of fainter objects. As we move later in the month, the Moon will move out of the way, but the sunrise will also be getting earlier, the sky will be getting brighter earlier, so you need to go out earlier to see the glow of our galaxy.

Moving along to just about 6 o’clock in the morning on the 18th gets the Moon and sunrise glow out of the way. We’ve got the glow of the Milky Way, and the different colors within in. Sometimes these colours are more obvious to the naked eye, other times it’s better to take a photograph. Whichever way, you should notice that down towards the southeast, the core of the galaxy has a sort of reddish glow, almost a reddish-orange, down near Scorpius. Out towards the arms of the galaxy we’re seeing a lot more of the pale, milky-blue color. In towards the center we’ve got that kind of orangey-red and out towards the edges we’ve got that kind of smoky blue color. Our galaxy is a galaxy, we’re a spiral galaxy, and as I mentioned in the recently, we are a barred spiral galaxy. To look at other galaxies recently, we took a look just between the tail of Leo the Lion and the head of Virgo the Maiden, just to see some of the galaxies that are actually quite near to us and our neighbours in the Local Group. For this example, we’re going to start by turning around to the north. At this time of year and in morning time the Plough or the Big Dipper is right above the North Star. We don’t even really have to look into the north, we could be facing south and just lean back to look up at the zenith.

Taking a closer look here at the handle of the Plough or the Big Dipper, using a binoculars or small telescope should let you catch a couple of those fuzzy shapes which we now know to be galaxies. If we take a closer look at the Whirlpool Galaxy, it’s a few fantastic example of a galaxy, known as a grand design spiral galaxy because of how obvious its arms and its shape is. This galaxy has more of a yellowy color towards the center. It’s got some dark dust lanes in its arms which also have that kind of darker, almost reddish color, but a lot of its arms are a wonderful blue. This blue color is usually revealed to be many blue spots through larger telescopes, and these blue dots are stars and star forming regions. The Whirlpool Galaxy is still a young enough galaxy to have enough hydrogen gas clouds to power the formation of stars, as well as having some dusty material. Dust scatters light the same way the air scatters light. When light gets scattered, you’re usually scattering out the higher energy wavelengths first. The higher energy wavelengths tend to bounce around and get thrown away by things that are in the way, like dust particles. Looking at the Pinwheel Galaxy on the other side of the Plough’s Handle, we’ve got plenty of those bright star forming regions again, and it still has that kind of yellowish color in towards the center.

There are other galaxies that we can find all over the sky, but we’re going to look for the ones that are close to Virgo and Leo, as they are some of the easiest to find and one of the riches areas of the sky to find them. As we start zooming in, we should start seeing some of those fuzzy spots, those are the galaxies that we are looking for. As there are so many galaxies in this region, we’re going to get a better idea of their variation and we’re going to get to see some spiral galaxies that are a little bit more on the orangey-red end of the spectrum. When stars form, there is often the formation of bright young blue stars and there is the formation of red stars, red dwarfs that last a very long time and over the course of their lifespan. Stars also age, they change from being young and bright and blue to being more old and red or orange. Blue stars with so much energy burn through their fuel very, very quickly, so in older galaxies we don’t see as much blue. Looking at M98, we’re not seeing a lot of blue at all, with the exception of a couple of scattered regions, the vast majority of the galaxy is an orangey-yellow kind of color. This is because most of the blue stars in that galaxy have died out, and there’s not enough hydrogen left to keep forming new ones. So although galaxies aren’t directly classified by their color, the color of a galaxy can tell us a lot about the galaxy and its history. Taking a look at M90, we’ve got a very similar effect going on, it’s got a yellowy color towards the center an almost a smoky gray towards the outside, but it doesn’t have those blue colors.

The fact that the arms of our galaxy are blue tells us that our galaxy is still producing blue stars, because blue stars don’t last very long. With other galaxies, if we’re seeing blue, we know that the galaxy is still producing new stars. Looking ar M58, it has clearly blue bands along its arms, so we know that this galaxy must have still been creating new stars within the last hundred thousand years or so. Otherwise the blue stars would have died, they would have collapsed, either gone supernova or nova, or just fallen apart depending on their size. Blue stars are often large enough to collapse and explode, but either way blue stars don’t last as long. When we see galaxies that are rich in those blue stars, we know that it’s still active or actively producing stars. Active galaxy usually refers to how active the black hole is in the center, but we know that the galaxy must still be forming new stars by their blue colour, regardless of whether their central black hole is swallowing up much material.

When we find yellowy-reddish colored galaxies, that’s an indication that the galaxy is no longer forming new stars. All the blue stars have burned out, and they haven’t been replaced by new star formation the way they have in the bluer galaxies. However, even the blue galaxies, even galaxies that very obviously have a lovely blue color, such as M91 a good example of a barred spiral galaxy, even with rich star formation in the arms, it still has that yellowy color towards the center. A lot of that is due to dust, because the dust in the center is able to scatter light and spread out the energy of the blue wavelengths. The blue wavelengths of light will lose energy from bouncing around, and they’ll shift to red. Not the same red-shift that we get from galaxies that are moving away from us very quickly, but the same kind of red shifting that we get when we see an orangey-yellow sunset. During the day, the sky looks blue, but as that light gets spread out more by our atmosphere, because it’s going through more atmosphere when it’s coming through at a lower angle, as it does at sunset, then we start getting more of these yellows and oranges. Looking at the Silver Streak galaxy, it doesn’t have loads of blue either, so there’s a good chance that that galaxy is at the older end after losing or using up a lot of its hydrogen.

Our galaxy follows the same rules as every other galaxy, the patterns that we see in other galaxies hold true for our galaxy as well. Coming back to morning time, that scattering effect is visible towards the center of our galaxy. Even though we still have areas where new stars are being formed, and if we take a closer look towards the core of our galaxy, there are nebulae where star formation is occurring, but there’s so much dust and gas that a lot of the color that we’re seeing is more of that reddish color. Out towards the arms of the galaxy, there’s still lanes of dust and gas, there’s still regions where light is getting scattered, but we’re also getting more of that bluish background color because there is still young blue stars forming in our galaxy, and there’s less dust and gas between us and the rest of the arms that are nearby. The color of galaxies is not normally a way that we divide up or classify the galaxies, but it does provide valuable information. These galaxies are different colors for particular reasons, and by looking at these colors, we can get a better idea of how that galaxy is behaving, perhaps how old that galaxy was, and in some cases even get an idea of how galaxies are interacting.

If hydrogen gas is being pulled out of a galaxy by its neighbor, very often that gas will be compressed and we’ll see it as a spate of new star formation. We might be able to see that, in fact, in the Whirlpool Galaxy, which is swallowing up one of its smaller companions, one of its satellite galaxies. There’s a lot of dust and gas being disturbed by this galactic cannablism, but we’re also seeing star formation along those dust lanes, and that’s just a galaxy swallowing up a smaller one. When two galaxies of a more equal size come together, the effect can be even greater. We’ll go with the Antennae Galaxy, which is a galaxy we looked at in the last piece where we looked at galaxies. It’s low to the horizon, so we’ll get rid of the atmosphere just to make the view a little bit clearer. There’s loads of blue in this fusing pair, loads of new star formation, because the hydrogen gas that was in these two galaxies is now getting smushed together by the collision of those galaxies, and that driving together of the hydrogen is spurring new star formation.

Seeing as we looked at the shape of galaxies in a recent piece, I thought it would be appropriate to take a look at the colour of galaxies in this piece, especially with our galaxy stretching across the sky in the morning. I hope you get to see our galaxy now that we’re in February and we’re at a good time for it to be seen, but of course if you have too much light pollution it won’t be visible. I hope that you enjoyed this piece, if you did, please do like it and if you enjoyed this kind of content, then please subscribe to this website and my YouTube channel. Thank you very much for watching and hopefully I’ll see you back here next time.