Along with transforming our view of the cosmos, the Hubble Space Telescope has changed the way astronomical data are compiled and shared. Details about every Hubble observation—including right ascension and declination of the target, observation wavelength, instruments used, and exposure time—are stored in the Barbara A. Mikulski Archive for Space Telescopes (MAST) database managed by the Space Telescope Science Institute (STScI) in Baltimore, Maryland. The accessibility of the data is largely responsible for the fact that, each year since 2003, more published research papers have been based on Hubble archival data than on new observations by the telescope.

Using the MAST data’s target classifications, which are provided by researchers after their proposals for telescope time have been approved, we broke down the variety of astronomical objects that Hubble has explored in nearly 550 000 scientific observations from 1990 through 2019.

Credit: Nadieh Bremer (Visual Cinnamon)

Each large collection of circles represents a primary classification category of a Hubble observational target. Each smaller circle denotes a descriptive keyword, or subcategory, from a prescribed list of terms associated with each category. Three pairs of MAST categories—Star and Ext-star (extragalactic star), Stellar cluster and Ext-cluster, and ISM (interstellar medium) and Ext-medium—each have identical sets of descriptive keywords, so those pairs were combined for the graphic. The smaller circles with a solid double outline denote keywords that can be paired with any categories. Because of the number of keywords, only some of the most common in each category are labeled.

The area of each smaller circle is proportional to the number of observations that were assigned a given keyword. Because observations in MAST can be tagged with one (or, in a small fraction of instances, two) of the primary categories and are supposed to have up to five keywords per category, any particular observation may be represented in many of the smaller circles within a category. Therefore, the relative size of the primary category groupings should not be read as representative of the distribution of observations among the targets.

The visualization reveals the diversity of objects that Hubble has looked at and the balance in target types that has been maintained over time. Below, we offer more detail on each of the eight categories.

Star and Ext-star

Star is the most commonly used target classification category in the MAST database. The category’s 100 prescribed keywords divulge information such as spectral classification, evolutionary state, and whether the star hosts planets.

The most common keyword is G V–IV, indicating the spectral class G for yellow stars and the luminosity classes V and IV for main-sequence and subgiant stars, respectively. In other words, stars similar to the Sun are a frequent Hubble target.

Since the discovery of extrasolar planets in the mid 1990s, Hubble has been increasingly staring at stars to characterize the planets around them. Exoplanet science now makes up about 20% of Hubble’s observation time, according to telescope mission head Tom Brown of STScI.

Also in this category are observations of targets like the remnant of Supernova 1987A (classification: Star: Supernova) and V838 Monocerotis (Star: Nova-like), a star from which a burst of light detected in 2002 illuminated the shell of gas and dust surrounding it. Hubble’s long lifetime, combined with the bounty of data in MAST, has allowed researchers to revisit such objects to examine changes in the systems over time.

Galaxy

The Galaxy category has 24 prescribed keywords that describe the galactic target, such as whether it’s spiral, elliptical, lenticular, or irregular.

One of the earliest projects Hubble undertook was to accurately measure the distance to the spiral galaxy M81 and from there to refine the estimate of the Hubble constant, which is tied to the rate of expansion of the universe. The observations allowed researchers to determine the periodicity—and thus the luminosity—of 32 Cepheid variable stars, which was used to calculate the distance to the galaxy as 11 million light-years (±10%). Previously, the periods of only two Cepheids had been measured in M81, and distance estimates ranged from 4.5 million to 18 million light-years.

Observations in 1994 of the nearby elliptical galaxy M87 revealed the first strong evidence of a black hole at the center of a galaxy. Subsequent observations have allowed astronomers to track a dynamic jet of high-energy electrons from the galactic center. More recently, Hubble has been used to examine the potential sources of gravitational waves detected by the Laser Interferometer Gravitational-Wave Observatory. In 2017 the telescope spotted the flare of light from the merger of two neutron stars in NGC 4993.

Stephan’s Quintet reveals the interactions of several neighboring galaxies. Near the center are two galaxies in the middle of a collision, with the galaxy to the upper right also being stretched by its neighbors. To their right is an area of star formation. The bluish galaxy to the upper left is actually seven times closer than the other galaxies and is not truly part of the rest of the group. Credit: NASA, ESA, and the Hubble SM4 ERO Team

Unidentified

The Unidentified category is often used for field observations in which there is no particular target. Other times Hubble is used to try to identify the unknown source of an electromagnetic emission detected by another observatory. Unidentified observations designed as blank, parallel, and high- or low-latitude fields cover a broad area and produce images with multiple galaxies or other objects. After the success of the 1995 Hubble Deep Field, the telescope was used for many additional such programs, such as Hubble Deep Field South (Unidentified: Parallel field) and Ultra Deep Field (Unidentified: Blank field, High-latitude field).

“Parallel field” is the single most used keyword among all the categories. When astronomers are using a Hubble instrument to look at a particular target, they (or another research team) can use one or more of the other instruments, which are pointed at different parts of the sky, to look at slices of space adjacent to the primary target.

The 1995 Hubble Deep Field is one of the telescope's most famous images. It is a composite of 342 separate exposures in UV, blue, red, and IR light taken over 10 days using the Hubble’s Wide Field and Planetary Camera 2. Capturing such faint, faraway galaxies helped push our knowledge of the universe's evolution farther back in time. Credit: R. Williams (STScI), the Hubble Deep Field Team, and NASA

Solar system

The prescribed keywords of the Solar system category pair the type of object with the target’s proper name (for example, “Planet Jupiter,” “Ring Saturn,” and “Asteroid Ceres”). For this visualization, we separated them by object type and proper name to better see the distribution of target types. As a result, the area of each circle with a proper name is roughly proportional to the number of total observations of that object.

Although Hubble’s images of Saturn and Pluto have been supplanted in the public consciousness by close-up shots from Cassini and New Horizons, its portraits of Uranus, Neptune, and the Jupiter system are still some of the best we have. Hubble observations have revealed previously unknown features such as four of Pluto’s moons and the presence of a subsurface ocean on Jupiter’s moon Ganymede.

Hubble is also one of the best sources for studying comets and asteroids. Images of the comet Shoemaker-Levy 9 (Solar system: Comet Shoemaker Levy 1991A1) and the impact marks it made when it collided with Jupiter in 1994 (Solar system: Planet Jupiter) represented one of the telescope’s banner moments in its first decade.

An X -shaped debris field trails behind an asteroid that had been discovered by a ground-based observatory in 2010. Researchers concluded that the debris was the result of a head-on collision between two asteroids. Credit: NASA, ESA, and D. Jewitt (UCLA)

Stellar cluster and Ext-cluster

Stellar clusters are grouped into globular clusters, open clusters, and associations—even looser collections of stars than open clusters—and the prescribed keywords for the category reflect those groupings. Clusters are studied primarily for understanding the evolution of stars and for measuring distances. Stars in a cluster were all born at roughly the same time, so variations found within a cluster are dependent on the stars’ masses.

The distance to the closest clusters, such as NGC 6397, can be measured using parallax. Knowing the distance, the stars’ luminosities can be determined, and those clusters can then be used, based on the ages of the stars within, as comparison points for other clusters. This allowed researchers to study NGC 346, an open cluster—located 210 000 light-years away in the Small Magellanic Cloud—that features extremely young stars not yet hot enough to fuse hydrogen in their cores.

Cluster of galaxies

The 12 prescribed keywords for the Cluster of galaxies category describe the size of the cluster and some key characteristics, such as high redshifts or the presence of an Einstein ring—in which the image of a distant object is warped into a circular shape around a massive intermediary object. Observations of lensing have been used to measure concentrations of dark matter, which makes up the majority of the mass of the cluster that causes the lensing.

Hubble has also examined gravitational lensing around galaxy clusters to increase the accuracy of measurements of dark energy. By determining the original shapes of lensed objects and combining that information with ground-based observations of the distance and speed of those objects, researchers were able to calculate the effect of dark energy on the lensing. The first such project looked at Abell 1689 (Cluster of galaxies: Rich cluster, Gravitational lens).

After the 1999 servicing mission, NASA used Hubble’s Wide Field and Planetary Camera 2 to capture the galaxy cluster Abell 2218, which also acts as a gravitational lens to reveal more distant galaxies. Credit: NASA, Andrew Fruchter, and the ERO Team (STScI)

ISM and Ext-medium

The interstellar medium is the dust, gas, cosmic rays, and radiation that fill the space between the stars. The prescribed keywords in the ISM category reflect the physical conglomerations of the physical material into clouds and nebulae, its temperature, and the absorption of light passing through the material.

The pictures Hubble has taken of planetary nebulae are some of the most well-known and stunning photos NASA has ever published.

The Eagle Nebula's Pillars of Creation were photographed by Hubble’s Wide Field and Planetary Camera 2 in 1995 (left) and Wide Field and Planetary Camera 3 in 2014 (center and right). The rightmost image is limited to the near-IR, revealing stars largely hidden by dust in the visible spectrum. Credits, from left: NASA, ESA, STScI, and J. Hester and P. Scowen (Arizona State University); NASA, ESA, and the Hubble Heritage Team (STScI/AURA); and NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Calibration

Despite our filtering of the MAST database to include only scientific observations, some of the observations that were initially made for calibration purposes were later reclassified. Many of them were of high enough quality that they were of potential value to researchers examining the Hubble database. The prescribed keywords usually indicate the nature of the calibration that was being performed (for example, “Astrometric,” “Narrow-band filter calibration,” and “Occultation-mode test”).

Soon after the 2009 servicing mission, a series of calibration photos of the Butterfly Nebula were taken using Hubble’s new Wide Field and Planetary Camera 3. Credit: NASA, ESA, and the Hubble SM4 ERO Team

Nadieh Bremer, a freelance data visualization designer/artist and astronomer based in the Netherlands, produced the visualization. Greg Stasiewicz, the web producer at Physics Today, wrote the article text.

The data presented in this article were obtained from the Space Telescope Science Institute’s Barbara A. Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc, under NASA contract NAS5-26555.