Tag Archives: Sagittarius A*

The Event Horizon Telescope

The Event Horizon Telescope (EHT) is a huge synthetic array for Very Long Baseline Interferometry (VLBI), which is created through the collaboration of millimeter / submillimeter wave radio telescopes and arrays around the world. The goal of the EHT “is to directly observe the immediate environment of a black hole with angular resolution comparable to the event horizon.”

The primary target for observation is Sagittarius A* (Sgr A*), which is the massive black hole at the center of our Milky Way galaxy. This target is of particular interest to the EHT team because it “presents the largest apparent event horizon size of any black hole candidate in the Universe.” The Sgr A* event horizon is estimated to have a Schwarzschild radius of 12 million kilometers (7.46 million miles) or a diameter of 24 million km (14.9 million miles). The galactic core (and hence Sgr A*) is estimated to be 7.6 to 8.7 kiloparsecs (about 25,000 to 28,000 lightyears, or 1.47 to 1.64e+17 miles) from Earth. At that distance, the Sgr A* black hole subtends an angle of about 2e-5 arcseconds (20 microarcseconds).

Another EHT target of interest is a much more distant black hole in the Messier 87 (M87) galaxy.

The member arrays and telescopes supporting EHT are:

  • Arizona Radio Observatory /Submillimeter Wave Telescope (ARO/SMT, Arizona, USA)
  • Atacama Pathfinder EXperiment (APEX, Chile)
  • Atacama Submillimeter Telescope Experiment (ASTE, Chile)
  • Combined Array for Research in Millimeter-wave Astronomy (CARMA, California, USA)
  • Caltech Submillimeter Observatory (Hawaii, USA)
  • Institute de Radioastronomie Millimetrique (IRAM, Spain)
  • James Clerk Maxwell Telescope (JCMT, Hawaii)
  • Large Millimeter Telescope Alfonso Serrano (LMT, Mexico)
  • The Submillimeter Array (Hawaii, USA)

The following arrays and telescopes are expected to join the EHT collaboration:

  • Atacama Large Millimeter / submillimeter Array (ALMA, Chile)
  • Northern Extended Millimeter Array (NOEMA, France)
  • South Pole Telescope (SPT, Antarctica)

Collectively, the arrays and telescopes forming the EHT provide a synthetic aperture that is almost equal to the diameter of the Earth (12,742 km, 7,918 miles).

EHT array sizeSource: graphics adapted by A. Cuadra / Science; data from Event Horizon Telescope

Technical improvements to the member telescopes and arrays are underway with the goal of systematically improving EHT performance. These improvements include development and deployment of:

  • Submillimeter dual-polarization receivers (energy content of cosmic radiation is split between two polarizations)
  • Highly stable frequency standards to enable VLBI at frequencies between 230 to 450 GHz (wavelengths of 1.3 mm – 0.6 mm).
  • Higher-bandwidth digital VLBI backends and recorders

In operations to date, EHT has been observing the Sgr A* and M87 black holes at 230 GHz (1.3 mm) with only some of the member arrays and telescopes participating. These observations have yielded angular resolutions of better than 60 microarcseconds. Significantly higher angular resolutions, up to about 15 microarcseconds, are expected from the mature EHT operating at higher observing frequencies and with longer baselines.

Coordinating observing time among all of the EHT members is a challenge, since participation in EHT is not a dedicated mission for any site. Site-specific weather also is a factor, since water in the atmosphere absorbs radiation in the EHT observing frequency bands. The next observing opportunity is scheduled between 5 – 14 April 2017. Processing the data from this observing run will take time, hence results are not expected to be known until later this year.

For more information on EHT, see the 2 March 2017 article by Daniel Clery entitled, ”This global telescope may finally see the event horizon of our galaxy’s giant black hole,” at the following link:


Much more information is available on the EHT website at the following link:


Radio telescope resolution

An article on the Las Cumbres Observatory (LCO) website explains how the angular resolution of radio telescopes, including VLBI arrays, is determined. In this article, the author, D. Stuart Lowe, states that “an array of radio telescopes of 217 km in diameter can produce an image with a resolution equivalent to the Hubble Space Telescope.” You’ll find this article here:


The Hubble Space Telescope has an angular resolution of 1/10th of an arcsecond (1e-1 arcsecond).

A VLBI array with the diameter of the Earth (1.27e+6 meters) operating in the EHT’s millimeter / submillimeter wavelength band (1.3e-3 to 6.0e-4 meters) has a theoretical angular resolution of 2.5e-5 to 1.2e-5 arcseconds (25 to 12 microarcseconds).

EHT should be capable of meeting its goal of angular resolution comparable to a black hole’s event horizon.

X-ray observation of Sgr A*

Combining infrared images from the Hubble Space Telescope with images the Chandra X-ray Observatory, NASA created the following composite image showing the galactic core in the vicinity of Sgr A*. NASA reports:

“The large image contains X-rays from Chandra in blue and infrared emission from the Hubble Space Telescope in red and yellow. The inset shows a close-up view of Sgr A* in X-rays only, covering a region half a light year wide. The diffuse X-ray emission is from hot gas captured by the black hole and being pulled inwards.”

This image gives you a perspective on the resolution of Sgr A* possible at X-ray frequencies with current equipment. EHT will have much higher resolution in its radio frequency bands.

NASA Sgr A* picSource: X-Ray: NASA/UMass/D.Wang et al., IR: NASA/STScI

More details on this image are available at the following NASA link:


Animation of Sgr A* effects on nearby stars

In my 24 January 2017 post, “The Black Hole at our Galactic Center is Revealed Through Animations,” I reported on how teams of astronomers were developing a better understanding of the unseen Sgr A* black hole through long-term observations of the relative motions of nearby stars that are under the influence of this black hole.

The Black Hole at our Galactic Center is Revealed Through Animations

Evidence is mounting that a supermassive black hole named Sagittarius A* (Sagittarius A star) dominates the center of our Milky Way galaxy. Long-term observations of the galactic center by teams of astronomers are refining our understanding of how stars move in relation to this unseen black hole.

European Southern Observatory (ESO) observations of the galactic center

The ESO, which has many observatories located high in the mountains of northern Chile, has a team involved in observing our galactic center. Two of the ESO optical observatories used in this effort are:

  • New Technology Telescope (NTT), at the La Silla Observatory, has a 3.58 m (11.75 ft) main mirror. In 1989, NTT became the first astronomical observatory with adaptive optics to help correct for atmospheric distortion.
  • Very Large Telescope (VLT), which consists of four Unit Telescopes with 8.2 m (26.9 ft) diameter main mirrors and adaptive optics. The telescopes can work together, to form a giant ‘interferometer’, allowing astronomers to see details up to 25 times finer than with the individual Unit Telescopes.

On 10 December 2008, ESO issued a “science release” entitled, ”Unprecedented 16-Year Long Study Tracks Stars Orbiting Milky Way Black Hole,” which summarized the results of observations made at NTT and VLT from 1992 to 2008. This study mapped the orbits of 30 stars in the region around the galactic center (and did not use VLT’s interferometric capabilities).

 Galactic center_eso0846aStars near our galactic center and the Sagittarius A* black hole. Source: eso0846 Science Release

The eso0846 science release is available at the following link:


In connection with this study, the ESO team also created a time-lapse video showing star motion around the Sagittarius A* black hole.

“Here, actual images, collected over the past 16 years, have been assembled into a time-lapse video. The real motion of the stars has been accelerated by a factor 32 million.”

This time-lapse video covers the central part of the above color image of the galactic center and shows stars moving around central point that is likely to be the black hole. You can see this animated sequence at the following link:


UCLA Galactic Center Group observations of the galactic center

The mission statement of the UCLA Galactic Center Group is:

“Transforming our understanding of Black Holes and their role in the Universe with high resolution observations of the Center of our Galaxy!”

The Galactic Center Group’s website is a good source of information on black hole science and the technologies employed to observe our galactic center. Their home page is at the following link:


The W.M. Keck Observatory on Mauna Kea in Hawaii is comprised of two large telescopes, each with 10 m (33 ft) main mirrors and adaptive optics. Currently the Keck Observatory has the largest optical / infrared telescopes in the world. These telescopes have higher resolution than ESO’s NTT and VLT.

Using images taken at the Keck Observatory from 1995 to 2014, the UCLA Galactic Center Group and the W.M. Keck Observatory Laser Team have released their determination of the orbits of stars within the central 1.0 X 1.0 arcseconds of our galaxy, as shown in the following diagram.


The team reported:

“These orbits provide the best evidence yet for a supermassive black hole. While every star in this image has been observed to move since 1998, estimates of orbital parameters are best constrained for stars that have been observed through at least one turning point of their orbits.”

This makes the star S0-2 especially important because it has been observed for more than one full orbital period, which for S0-2 is only 16.17 years. The team estimates that the Sagittarius A* black hole has a mass of 4 million times the mass of the Sun.

The UCLA Galactic Center Group and the W.M. Keck Observatory Laser Team have created a series of animations that demonstrate the motion of stars near the Sagittarius A* black hole. You can navigate to these animations from the home page listed above or use the following direct link:


The three animations showing star motions around the Sagittarius A* black hole are:

  • Animation of the Stellar Orbits around the Galactic Center
  • 3D Movie of Stellar Orbits in the Central Parsec
  • Sagittarius A* – IR (infrared)

The importance of adaptive optics is astronomical observations is demonstrated in another animation from the UCLA Galactic Center Group.

“This animation shows observations of the Galactic Center with and without adaptive optics, illustrating the resolution gain. Adaptive optics corrects for the blurring effects of the Earth’s atmosphere. Using a bright star, we measure how a wavefront of light is distorted by the atmosphere and quickly adjust the shape of a deformable mirror to remove these distortions.”

Screenshots from this animation are shown below. The screenshot on the left is with adaptive optics OFF. The image on the right is with adaptive optics ON.

Adaptive optics OFF  Adaptive optics ON

The future

In my 6 June 2015 post, “Three Very Large, New Optical Telescopes are Under Development,” I reported on the Thirty Meter Telescope (TMT), which originally was planned for construction on Mauna Kea, near the Keck Observatory. As the name implies, TMT will have a 30 m (98.4 ft) main mirror and adaptive optics. To illustrate the improved resolution of TMT, the UCLA Galactic Center Group developed an animation showing Sagittarius A* images for the following three cases:

  • Keck telescopes with current adaptive optics (AO)
  • Keck telescopes with “next generation” adaptive optics (NGAO), and
  • The future TMT with adaptive optics.

As you can see in the following screenshot from this animation, the expected results from the much higher resolution TMT quite impressive.

Relative resolution power - Keck & TMT

TMT’s actual construction site is being reconsidered and construction has been delayed. However, ESO has broken ground for the even larger European Extremely Large Telescope (E-ELT), which is being built now at Cerro Armazones, Chile. This giant telescope has a 39 m (128 ft) main mirror and adaptive optics. It will become the largest optical / infrared telescope in the world when it is commissioned as part of ESO’s Paranal Observatory in 2024. Hopefully, time on this great telescope will be allocated to observing our galactic center.