HST Scorecard

Anticipated Science

Refine the Cosmic Distance Scale and Hubble Constant

With Hubble data, the expansion rate of the universe (H₀) has been determined to better than 2.2% accuracy (astonishing since 10% was the goal), corresponding to an age for the universe of 13.8 billion years. This refinement of H₀ produced a measurement of cosmic acceleration and inferred the existence of dark energy. Most recently, the refined data on stellar mileposts made with Hubble appears inconsistent with the observations of the early universe with European Space Agency’s Planck mission. It is unclear what sort of physics is required to explain this discrepancy.

Formation and Processes in Galaxies

The detailed formation of galaxies has been and continues to be an important cornerstone of astrophysical studies. The deep imaging and high-resolution capability of Hubble has been used to probe the nature of supermassive black holes in galaxies, including contributing to studies of emission from active galactic nuclei (AGN) and their relationship to galaxy formation early on. The understanding of the host galaxies of quasars (QSOs), AGN, and the supermassive black holes proposed to be responsible for high-energy output from the centers of galaxies relied on Hubble’s stable diffraction limited optics, high-contrast imaging, and spectroscopy at the diffraction limit. In addition, the technique of using absorption lines produced by QSO light passing through the intergalactic medium and the interstellar medium was probed with Hubble UV spectroscopy.

Intrinsic Brightness of Supernovae

Supernovae, besides being dramatic, catastrophic events for a massive star, can be used to measure the expansion rate of the universe. Supernovae of various types are studied to understand the dramatic end phases of massive stars and how mass loss during those event returns chemically entriched material to the interstellar medium. Type Ia supernovae, being a known intrinsic brightness, were observed using Hubble’s high resolution, deep imaging capability for objects out to great distances to verify their applicability as standard sources for measurement. These supernovae contributed to the discovery of cosmic acceleration and thus, dark energy. Hubble data is essential for measuring these supernovae because they are so distant and faint and cannot easily be measured with other techniques.

Probing the Origin and Evolution of the Solar System

Hubble observations of solar system objects exquisitely observed in the UV as well as the optical infrared have revealed auroras and rings for the large gaseous planets, the weather system on Mars, new moons of Pluto, moons of dwarf planets, comets and asteroids in collision and breaking apart due to rotation, observations of Kuiper belt objects in the distant solar system, volcanic emission from Jupiter’s moon Io, and water plumes from Europa, and another Jovian moon. Hubble data has also supported numerous planetary missions such as the Mars rovers and orbiters, Juno, Cassini, and New Horizons.

Unforseen Discoveries

Dark Energy

The existence of dark energy was proposed as a consequence of the discovery of the acceleration of the universe and the inference that the age of the universe is 13.8 billion years old. Hubble observations were and continue to be essential to refining the precise determination of the expansion rate of the cosmos, the acceleration, and nature of the associated dark energy component and any discrepancies with other studies of the events defining the origin of the universe.

Deep Fields

Hubble observational campaigns have provided some of the deepest images of the cosmos both in deep fields and through the use of gravitational lenses. The Hubble Deep Field, the Hubble UV Ultra Deep Field and the Frontier Fields (gravitational lenses in selected clusters), probe the formation of the universe, galaxy assembly early on and the birth of stars, along with the exotic phenomena such as active galactic nuclei, formation of supermassive black holes and quasi-stellar objects.

Galaxy Collisions and Mergers

Observation of galaxies are a staple of Hubble observations and surveys have shown that galaxy collisions and tidal interactions are common and related to the conditions that spawn star formation, specifically the formation of star clusters. These collisions, viewed through the exquisite imagery of Hubble, appear to be implicated in the formation of supermassive black holes and active galactic nuclei. Some of these collisions have helped probe the nature of dark matter – prevalent in galaxies yet not completely understood.

The Milky Way

Hubble observations of the center of our Milky Way galaxy, coupled with observations from other observatories, allowed the mapping of the motions of stars circling the supermassive black hole there. The stars belong to a massive cluster at the nucleus near the galaxy’s central black hole being studied to understand how such clusters form. The high spatial resolution of Hubble allowed observations of the crowded bulge of the Milky Way, indicating the stellar populations there. Spectroscopic observations of distant QSOs through the Milky Way showed a balloon-like structure emanates from the center, perpendicular to the galaxy’s disk. Hubble data suggests that explosions from the central black hole have taken place in the past.

Black Holes

While Hubble was destined to observe the environments of the central black holes in galaxies, numerous studies revealed that black holes come in wide variety of masses including supermassive black holes in galaxy centers, massive black holes in smaller galaxies, down to individual stars that have totally collapsed into black holes as well. Therefore, the sizes of black holes have been demonstrated through Hubble data to exist over many orders of magnitude.

Exoplanets and Disk Systems

Early on, the high spatial resolution of Hubble images for the Orion Nebula provided clear evidence that protoplanetary disk systems commonly formed around stars. Later observations of the star Fomalhaut showed a clear circumstellar dusty disk through use of a coronagraph, blocking the star’s light to enhance detection of the disk. Subsequently Hubble data indicated that the disk contains at least one planet.

Hubble data contributed to the discovery of extrasolar planets through transits, paving the way for further studies by space telescopes. In addition, spectroscopic observations with Hubble of extrasolar planets and their parent stars revealed compositions such as methane, titanium oxide and water, the wide variety of atmospheres with and without clouds, and atmospheres of planets being ripped by close approach to the central star in the system.

Star Clusters

A fundamental key in understanding galaxies and the evolution of universe is the study of star clusters. Star clusters are predominant especially in spiral galaxies, reside in the cores of massive galaxies and comprise the oldest star clusters, globular clusters, which are remnants from the early universe. Hubble observations have traced the chemistry of star clusters, provided constraints on theoretical models, illustrated the dynamics of stellar nurseries, and delineated the aftermath of vigorous interactions between galaxies. The distribution of globular clusters also has been used to measure the mass of the Milky Way galaxy.

Solar System

In addition to providing dramatic global views of planets, their weather systems and aurora, Hubble imagery has captured events such as collisions of asteroids/comets into Jupiter, disintegrating comets, and the shape of Pluto’s moons, Nix and Hydra. Hubble documented the numerous moons of Jupiter and Saturn, and further observations of the distant dwarf planets indicated that these objects too have moons. Hubble’s high resolution has been used to observe the structure of comets as they trek from the outer solar system in towards their encounter with the Sun.