If someone comes and tells you that love is in the air, show them this image and say, “forget air, love is in the universe.” This cosmic structure is known as the Heart Nebula. It is one of the most beautiful things in the universe, and it shows that nature is much more of an artist than a scientist or an engineer. But what it really is? How did this thing form, Why is it colored red, and what’s happening in this region of the universe? Too many questions but don’t worry. You don’t have to be an expert in astronomy to understand all this.
How was this nebula discovered?
Also known as IC 1805, the Heart Nebula was discovered more than 200 years ago, in 1787, by astronomer William Herschel. He was one of the most influential astronomers in the history of humankind. In 1774, he constructed his own giant telescope and started observing the objects in the night sky. He was the one who discovered the seventh planet of the solar system, Uranus.
On the night of November 3, 1787, Herschel pointed his telescope towards the constellation of Cassiopeia, a W shaped constellation through which a rich section of the Milky Way runs. After making several observations, he came across a fuzzy patch in the sky. He indexed the nebula in his catalog that contained over 5000 astronomical objects.
Since the resolving power of Herschel’s telescope was small, he could not make much out of it. As astrophotography improved in the next couple of centuries, scientists studied the nebula in detail and captured better pictures of this cosmic structure. While the nebula’s official designation is IC 1805, it became popular in the science community by the name of Heart Nebula because its shape resembles the human heart, as depicted in popular media.
How big is the Heart Nebula?
The Heart Nebula has a diameter of 330 light-years, where one light-year is roughly 6 trillion miles. To put that into perspective, if you are traveling in the fastest spacecraft that can go at 42 miles per second, it will take 1.3 million years to travel across this nebula.
Also, the Heart Nebula lies 7200 light years away. So even light takes 7200 years from the nebula to reach us. This means the image of the nebula that we see today is how it looked 7200 years ago! Before I tell you why it’s glowing red, let me first introduce the concept of nebulae in astrophysics. I’ll give an overview. You can read the detailed article here.
What is a nebula?
In astronomy, a nebula is an interstellar region of dust and gas, primarily composed of the first two elements of the periodic table, hydrogen, and helium. Depending on the location of the nebula, there might be trace amounts of other chemical elements. There are different kinds of nebulae in the universe, but they are broadly connected with stars’ birth or the aftermath of a dying star.
When a Sun-like star reaches its deathbed, it expels its cool outer layers. This is known as a planetary nebula. The name, however, is a misnomer. It has nothing to do with the planets. When a dying star undergoes a titanic explosion, what is left behind is a supernova remnant. Hence, these two types of nebulae are related to a star’s death.
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But most of the nebulae in our galaxy are the cradles of stars. They are stellar nurseries rich in hydrogen and helium, the two main ingredients of star formation. Some of these star-forming nebulae glow or reflect light, while the others appear as dark patches in the sky obscuring the background light. The Heart Nebula is an emission nebula where new stars are taking birth. You may be wondering why the nebula is red in color. The answer to this question lies in the hydrogen atom, but before that, let me introduce one small concept related to the stars.
According to a recent estimate made by the European Space Agency, over 1 trillion trillion (1024) stars exist in the universe! Astrophysicists broadly classify the stars into 7 main categories, according to their surface temperature. The O and the B type stars are the hottest, with surface temperatures reaching 25,000 Kelvins. They are followed by the A, F, G, K, and M type stars with decreasing surface temperatures. At about 6,000 kelvin, our Sun is a G type star. Notice how the stars’ apparent color changes as we go from the O type to the M type. With this changing color, the type of radiation that is being emitted by these stars also changes.
The secret of the red glow
The young O and the B type stars emit copious amounts of ultraviolet radiation. These high energy UV photons emitted by the OB stars ionize the neutral hydrogen atoms present in the nebula. Ionization means that the hydrogen nucleus and the electron orbiting it become separated. They reassemble, but this time in an excited state.
The captured electrons cascade down through the quantum states of the hydrogen atom emitting radiation or a spectral line. One of the most significant jumps of the electron is from the third to the second energy level or the Balmer series’s first line (the smallest green line in the illustration). This jump creates a spectral line having a wavelength of 656.28 nanometers.
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This spectral line is known as the H Alpha line – and is one of the most important spectral lines used by astronomers. If you look at the electromagnetic spectrum, this wavelength lies in the red end. This is why most of the emission nebulae are red in color. Back on Earth, the detected wavelength doesn’t need to be exactly 656.28 nanometers. It might be redshifted or blueshifted, depending on the relative motion of the nebula. From the calculated redshift, we can also determine how fast it is moving away from us.
As far as the Heart Nebula is concerned, its glow comes from an open cluster of young supermassive stars called Melotte 15. Lying near the center of the nebula, the stars in this cluster are supermassive O and B type stars, some of them 50 times bigger than the Sun. On cosmic time intervals, they are young and formed about half a million years ago. Observations show there also used to be a radio-emitting microquasar in the region, which was expelled millions of years ago.
An interesting neighborhood
The Heart Nebula has an interesting neighborhood. Just to the left of the nebula lies another cloud of gas called the soul nebula. The Heart and the Soul Nebula are home to seven clusters of young stars. In this image, you can see there are two background galaxies. These galaxies are difficult to spot in the optical image but become visible in infrared observations. One of them is an elliptical galaxy, and the other is a lenticular galaxy. The famous Andromeda galaxy also lies in the neighborhood of the Heart Nebula.
Learn astrophysics at home
Did you always want to learn how the universe works? Read our 30-article Basics of Astrophysics series absolutely free of cost. From the popular topics such as stars, galaxies, and black holes to the detailed concepts of the subject like the concept of magnitude, the Hertzsprung Russell diagram, redshift, etc., there is something for everyone in this series. All the articles are given here. Happy reading!