To Catch a Star

Prior to the New Horizons flyby this month, the best image we had of Pluto came from the Hubble Space Telescope:

Credit: NASA, ESA, and M. Buie (Southwest Research Institute)

This struck me as being very similar to our best image of a stellar surface besides the Sun.  Very few stars have had their surfaces resolved.  Most stars appear as points of light in even the largest of telescopes.  Below is an image of Betelgeuse taken by the Hubble:

Credit: Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA

So the question to ask is, how can we obtain high resolution images of stellar disks as we did with Pluto?  Barring radical advancements in spacecraft propulsion, sending a mission to a star is not feasible.  The nearest star, Proxima Centauri, is 4.24 light years away from us.  The New Horizons probe, which is traveling at 35,000 mph (56,000 km/hr), would take over 100,000 years to reach that destination.  Obviously, the solution is constrained to be Earth bound or near Earth technology.  We need to build telescopes with higher resolution.

The theoretical resolution capability of a telescope is measured by the following equation:

R = 1.22(λ/d)

Where R is resolution in radians, λ is light wavelengths, and d is the diameter of the telescope’s primary mirror.  The smaller the value R, the higher the resolution and the greater the ability of a telescope to resolve finer detail.  Thus, to improve resolution, we’re gonna need a bigger telescope.

The other factor to consider is a telescope’s light gathering ability.  This is directly related to the area of the primary mirror.  If the mirror is circular in shape, this is described as:

A = πr2

Where A is area, π is the constant pi (about 3.14), and r is the radius of the mirror.  The larger A is, the larger a telescope’s ability to collect light.  Thus, a larger telescope can detect dimmer objects in the sky.  It can also detect objects farther away from Earth.  A mirror with a radius of 1 meter has an area of 3.14 square meters.  A mirror with a radius of 2 meters has an area of 12.56 square meters.  In other words, doubling the radius increases the area by four times.  Thus, what a mirror with a radius of 1 meter (diameter of 2 meters) can see 10 light years away, a mirror with a radius of 2 meters (diameter of 4 meters) can see 40 light years away.

To sum up the above, mirror diameter resolves finer detail on an object while surface area will enable more targets to be imaged.

As you might gather from the above, the best stellar targets to resolve would be stars that are very large (and hence, very bright) and relatively close.  And that is why Betelgeuse was the first star to be resolved in 1995 by the Hubble.  Betelgeuse is a red giant whose diameter could fit the orbit of Mars.  It is fairly close at 642 light years.  However, observing time is scarce on the Hubble as competition is fierce to use it.  Utilizing surface observatories can open up more opportunities for this line of research.

Besides the Hubble, another telescope to resolve a stellar disk is the CHARA array at Mt. Wilson.  CHARA relies on a technique referred to as interferometry.  An array of telescopes can have the same resolving ability of a single telescope the same size.  This is dependent upon the lightwaves received from all the telescopes in the array to be synched in phase together.

Credit: NASA

Radio astronomy was the first to use interferometry.  Radio waves are much larger than light waves.  In fact, radio waves can be several kilometers long.  Hence, it was easier to synch radio waves together than optical lightwaves.  The Very Large Array (VLA) in New Mexico uses this technique.  If you seen the movie Contact, you may be familiar with the VLA.  The VLA is an array of 27 radio antennas that in its widest configuration can provide the same resolution as a single antenna 22 miles (36 kilometers) wide.

Image courtesy of NRAO/AUI

Both economics and physics play a role here.  It is less expensive and structurally easier to build an array of small antennas rather than build a single dish 22 miles wide.  The tradeoff is gaining resolution at the cost of collecting ability.  The VLA does not collect the same amount of radio waves as a single 22 mile wide dish.  This plays a role in optical interferometry as well.

Optical interferometry presents a couple of challenges radio interferometry does not.  Visible light waves are disturbed by atmospheric turbulence, that is what causes the twinkling you see when you look at the stars at night.  The other is light waves are much smaller than radio waves.  In fact, radio waves are 1,000 – 1,000,000 times longer than visible light waves.  As a result, optical interferometry took longer than radio interferometry to develop.  Adaptive optics solved the turbulence problem.  Computer guided optical tracks solved the problem of combining very small visible light waves.

Credit: MRO/NMT

In the above example, light from the star travels a longer path to telescope 1 than to telescope 2.  Consequently, the light from telescope 2 must take an equally longer optical path before it is combined with the light from telescope 1 to ensure both are in phase together.  If the optical path is not calibrated correctly, the waves will arrive out of phase such as below:

Credit: Wiki Commons

The optical path is adjusted so that the red wavelength travels the distance θ longer than the optical path traveled by the blue wave so both waves are in synch and are combined where the peaks and valleys of the wave match up to produce a sharp image.

The CHARA interferometer currently has the longest baseline with six 1-meter telescopes spread out with a diameter of 330 meters.  The CHARA is located adjacent to the 100-inch telescope at Mt. Wilson where Edwin Hubble made his historic discovery of the expanding universe.

In 2007, the first resolved image of a disk from a near sun-like star was produced by the CHARA.  The star was Altair located 17 light years from Earth.  The image is below:

Credit: CHARA/GSU

Altair’s differs from the Sun in that its rotation rate is much faster.  The Sun rotates once every 25 days.  Altair rotates once every 9 hours.  Consequently, Altair is not spherical but bulges out from its equator.  This also causes Altair to have a cooler temperature at the equator and is thus darker in those regions.  This is the type of detail we can obtain when stars can be resolved as disks rather than points of light.

So what does the future hold?  Can future telescopes provide high resolution detail of stellar surfaces just as New Horizons provided for Pluto?  What should we hope to find if we can?  Stars, like planets, come in many sizes with differing physical characteristics.  The only star we can see with fine detail is the Sun.  The Solar Dynamics Observatory (SDO) images the Sun in both visible and ultraviolet light.  This video is an excellent overview of the solar surface detail provided by the SDO.

Given that attempts to resolve stars light years away from Earth is still in its nascent stage, it will be many years before we achieve that kind of quality imaging.  Nonetheless, there are promising developments that should improve our ability to acquire more detail of the features on stellar surfaces.

The first is a new generation of 30-40 meter telescopes set to go online in the 2020’s.  Combined with adaptive optics technology, they will have resolution on the order of 10-12 times that of the Hubble Space Telescope.  That, along with their immense light collecting ability, should improve the amount of suitable stellar targets to resolve.

The second, is the Magdalena Ridge Observatory Interferometer (MROI) in New Mexico.  The MROI will consist of an array of ten 1.4 meter telescopes with a maximum baseline of 1115 feet (340 meters).  The anticipated resolution will be 100 times that of the Hubble.  No definite timeline is given for completion of the MROI as it is a matter of obtaining funding to continue construction.  However, it is partially completed and hopefully will be up and running within ten years.

Credit: MROI

The next logical step is to build an optical interferometer in space.  Since 2000, NASA has been working on preliminary plans for the Stellar Imager (SI) mission.  The SI would consist of an array of 20-30 one meter mirrors with a baseline of 0.5 km (1,640 feet).  The array would be located in the Lagrange L2 point.  The L2 point is a gravitationally stable region 1.5 million km (1 million miles) from Earth.  This is also where the James Webb Space Telescope will reside a few years from now.

Credit: NASA/GSFC

Why should we endeavor to resolve stellar surfaces?  This is key to understanding the magnetic activity of a star and the stellar wind that emanates from the star towards orbiting planets.  These results can be integrated from the findings of exoplanet research.  Questions to be answered are, does the star generate space weather that could prevent life from forming on orbiting planets?  How strong a magnetic field does an orbiting planet require to protect life from local space weather conditions?  Does the star’s magnetic and irradiance cycle oscillate  in a manner that might inhibit the formation of life in its planetary system?  Detailed information from  main sequence stars older than the Sun could give us a look at the future of the Sun and how its evolution will impact life on Earth.

The SI is currently projected to start sometime around 2025-30.  More than likely, it will be later than that.  That is not unusual for cutting edge space missions.  The concept for an orbiting telescope was first proposed by Lionel Spitzer in 1946.  It took 44 more years to become reality as technical, funding, and political hurdles had to be overcome culminating with the launch of the Hubble Space Telescope in 1990.

I may never see the SI become reality in my lifetime, but it is important for my generation to lay down the ground work for our children to benefit from the results.  Astronomy, just like the building of the great cathedrals, can often be a multi-generational effort.

*Image on top of post are stars resolved by CHARA.  These stars are rapid rotators whose swift spinning motion flattens out their shape at the equator.  The images are false color and are designed to demonstrate temperature gradients on the surface of each star.  The brighter the color, the hotter the star is at that region.  The arrow on top indicates the true color of each star.  Credit:  CHARA/MIRC

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