The Hubble Space Telescope has been photographing the universe for over two decades with a gallery of stunning imagery to show for it. But what are we looking at in these images, exactly?

Take a tour of 10 of the HST’s most beautiful photos with us as we turn to science to explain just what it is you’re seeing in each one.

First, a note on color. Were we to take a spaceship and fly out to these sights, they would not appear the way we see them in these photos. In fact, the pictures the HST takes are in black and white. So where does the color come from?

Every HST photo is a composite of multiple photos – typically two or three – each taken to observe a different wavelength of light. The HST can detect not only the visible spectrum of light, but also light that our eyes cannot perceive – ultraviolet and infrared. In this way, the HST is able to visually depict what would otherwise be invisible to us.

A color is assigned to each black and white photo, generally selected to highlight noteworthy features within an image, and the final composition comes together like a full-color photo, just like how a computer monitor combines red, green, and blue to produce what you observe on-screen. So while these images aren’t exact representations of what you would see if you viewed these galactic beauties yourself, the use of color is only there to bring out their complexity and wonder.

Click on to see some of the most beautiful images from space ever taken.

pillarsofcreation

Our first stop on our tour of the universe is to the Eagle Nebula and its famous Pillars of Creation, as seen in some of the HST’s most iconic imagery. What we’re looking at in this picture are columns of cold hydrogen gas and dust that are sticking out of a dark molecular cloud, like a butte in Utah’s Monument Valley sticking out of a flat desert plain. In a similar manner that buttes form by resisting the erosion that wears away the surrounding rock, these columns are more resistant to the erosion caused by the intense ultraviolet light blasting from nearby massive, newborn stars.

horsehead

Next, we visit the Horsehead Nebula, which appears dark and shadowy in visible light, like an ebon knight on a cosmic chess board. However, when viewed in infrared as in this image, we observe a far more detailed and Giger-esque picture painted against a backdrop of distant galaxies and the stars of the Milky Way.

UV light is blasting in from above and has eroded the surrounding gas clouds, but the ridge of this horse’s head has a greater density of material – hydrogen and helium laced with dust, in this case – which makes it more resistant to the weathering effect. Like an umbrella, this ridge protects the material below it from the incoming light.

sharpless

Onward to the star-forming region known as the Sharpless and one of its many beautiful sights: the Celestial Snow Angel. At the center of this nebula is a young and massive star firing jets of hot gas from its poles – the glowing blue wings in this image. The vertical strip that divides the wings is a ring of dust and gas orbiting the star. The dark red veins that encircle the wings form the nebula itself, clouds of dust illuminated by the star.

mysticmountain

Second star to the right, and straight on ’til morning, and we’ve arrived at the Carina Nebula’s Mystic Mountain. We see another pillar – or mountain – of gas and dust being eroded by UV light, but this time, there’s a twist: infant stars within the mountain’s peaks are firing off jets of hot gas in pairs that point in opposite directions. Two of these jet pairs are visible; one is prominently displayed at the mountain’s highest peak, while the other is to the lower left of the first, toward the center of the image. A haze of gas and dust illuminated by starlight surrounds the mountain.

butterfly

Our next stop on this tour is one of the hottest stars in the galaxy and its surrounding Butterfly Nebula. While similar to the Celestial Snow Angel, this nebula’s structure is one of the most complex ever observed. The central star, a white dwarf, is obscured from our view by a dense, doughnut-shaped ring of dust that forced the material spewing out of the star into an hourglass shape.

The butterfly’s wings consist of superheated gas expelled from the star in phases as the star evolved throughout its life and cast off its outer layers. The projections pointing back toward the star may represent denser material that was more resistant to the stellar wind – not unlike the Pillars of Creation.

antennae

While the scale of the images we’ve observed so far have already been – wait for it – astronomical, this next sight dwarfs them: the two Antennae Galaxies colliding into one another. The large orange spots are the cores of the galaxies crisscrossed by brown filaments of dust. Blue star-forming regions are surrounded by glowing hydrogen gas, shown in pink.

This image serves as an ominous forecast of what will likely one day happen when the Milky Way and Andromeda Galaxy collide.

dustyspiral

From a scene of chaos, let’s turn to one of serenity. Spiral Galaxy NCG 4414 may not have a catchy name, but its well-defined spiral arms are almost hypnotizing. The shift from warm to cold colors from the galaxy’s center to its edges is a reflection of the ages of the stars within. Within the outer arms, we see blue regions filled with young and forming stars, whereas moving toward the center, we see older stars, mostly yellow and red in color. The dark streaks observed in the arms are interstellar dust silhouetted against the starlight.

spiralnebula

Zooming in on a smaller – but no less impressive – object, we stop next at the Spiral Planetary Nebula. Bright and gaseous, this nebula exhibits at least two distinct features: the orange material being blown away in a backwards s-shape, and the blue material that is like a roiling cloud expanding more uniformly.

When a star is dying, it blows off layers of gas in different stages, and the different patterns we see in this image are the result of this process of ejecting material at different times and in different directions. The glow is a product of the radiation given off from the remains of the star at the center of the explosive structure: a white dwarf.

tarantula

Arachnophobes, beware – we’ve arrived at the Tarantula Nebula, so named for its glowing filaments that vaguely resemble spider legs. The largest and most prolific known star-forming region in the Milky Way, this nebula is home to the most massive stars ever observed and is illuminated by a tight star cluster left of center in the image. The surrounding cloud of hydrogen in which the stars were born – much like the amniotic sac in which a fetus develops – is being carved into intricate shapes by the intense UV radiation from the hundreds of thousands of massive stars.

stellarspire

We’ll end our trip where we started – a return to the Eagle Nebula. But this time, rather than mighty pillars, we’re visiting the delicate Stellar Spire, a wispy plume of gas and dust. Just like the Pillars of Creation, the Stellar Spire consists of cold hydrogen sculpted by the UV light of massive, young stars. The visible haze around the pillar is gas boiling off the pillar’s surface, and the dark shape near the plume’s top is a denser hydrogen cloud that is resisting the UV erosion. The spire appears backlit against warmer gas clouds in the distance.

This is just a small glimpse of the universe provided by the Hubble Space Telescope. An international team of scientists just brought a new generation of telescopes online last year. And just last year, one of the oldest known stars was discovered. Which is all to day, we can’t wait to see what’s next.

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