Do stars made of antimatter exist in the universe? Possibly.

One of the great mysteries of cosmology and astrophysics is that even though equal quantities of matter and antimatter appear to have been produced during the “Big Bang”, today there is only a negligible quantity of antimatter in the observable universe. We do not appear to live in a matter-antimatter symmetric universe.

If antimatter stars, “antistars”, do exist, how could we distinguish them from stars made of normal matter? The light emitted from an antistar would look identical to the light emitted by a normal-matter star.

But if normal matter were infalling upon an antistar, the contact between matter and antimatter would generate an annihilation spectrum of gamma ray photons that peaks around energy 70 MeV (half the mass of a neutral pion) up to a sharp cutoff around 938 MeV (mass of the proton).

A recent analysis of data collected by the Fermi Gamma-ray Space Telescope found fourteen possible antistars. These fourteen point sources produce a gamma-ray signature indicative of matter-antimatter annihilation.  These point sources do not exhibit the characteristics of other known gamma-ray sources.  For example, they are not, ostensibly, pulsars, active galactic nuclei, or black holes.

The positional error ellipses for these fourteen point sources range from 11×10 arcminutes up to 128×68 arcminutes (95% confidence). Here are optical images of these sources from the Palomar Digital Sky Survey, in order of right ascension (epoch 2000 coordinates).

4FGL J0548.6+1200
5 48 38.8 +12 00 10
29.6’×23.6′ error ellipse
field of view 48.5′, Orion
bright star near crosshairs is HD 38797
4FGL J0948.0-3859
9 48 03.6 -38 59 57
53.7’×45.9′ error ellipse
field of view 48.5′, Antlia
bright star near crosshairs is TYC 7693-3238-1 ;
nebulous streak through the field is unidentified, 11˚ from the galactic plane
4FGL J1112.0+1021
11 12 03.1 +10 21 31
128.3’×67.9′ error ellipse
field of view 1.63˚, Leo
brightest star in field is HD 97502
4FGL J1232.1+5953
12 32 06.1 +59 53 03
15.4’×13.0′ error ellipse
field of view 24.11′, Ursa Major
brightest star in field is TYC 3847-229-1 ;
the galaxy is LEDA 2595040
4FGL J1348.5-8700
13 48 30.7 -87 00 47
10.6’×9.7′ error ellipse
field of view 11.99′, Octans
4FGL J1710.8+1135
17 10 50.5 +11 35 57
30.7’×26.7′ error ellipse
field of view 48.49′, Ophiuchus
brightest star near crosshairs is HD 155411
4FGL J1721.4+2529
17 21 24.7 +25 29 25
36.4’×25.2′ error ellipse
field of view 48.49′, Hercules
brightest star in field is HR 6455
4FGL J1756.3+0236
17 56 21.2 +02 36 52
19.0’×14.1′ error ellipse
field of view 24.11′, Ophiuchus
4FGL J1759.0-0107
17 59 03.7 -01 07 11
25.7’×22.8′ error ellipse
field of view 24.11′, Serpens
brightest star in field is HD 163914
4FGL J1806.2-1347
18 06 14.7 -13 47 36
19.2’×11.5′ error ellipse
field of view 24.11′, Serpens
4FGL J2029.1-3050
20 29 09.6 -30 50 06
31.0’×21.4′ error ellipse
field of view 48.49′, Microscopium
brightest star in field is HD 194640
4FGL J2047.5+4356
20 47 32.0 +43 56 33
58.9’×34.0′ error ellipse
field of view 1.63˚, Cygnus
brightest star in field is 56 Cyg ;
behind it is the Pelican Nebula (IC 5070)
4FGL J2237.6-5126
22 37 39.4 -51 26 05
20.7’×16.8′ error ellipse
field of view 24.11′, Grus
brightest star near crosshairs is TYC 8452-1160-1 ;
the edge-on galaxy is LEDA 92766
4FGL J2330.5-2445
23 30 35.6 -24 45 15
28.5’×20.5′ error ellipse
field of view 48.49′, Aquarius
brightest star near crosshairs is HD 221258

Since there appears to be no known way to distinguish a star made of antimatter from one made of matter—except for the gamma-ray signature of matter infalling onto the antimatter star, a higher-resolution gamma-ray telescope or interferometer (10 – 1000 MeV) needs to be developed to localize these candidate sources to within a few arcseconds. Higher spectral resolution will help as well, allowing a more detailed characterization of the gamma-ray spectrum.


S. Dupourqué, L. Tibaldo and P. von Ballmoos. Constraints on the antistar fraction in the solar system neighborhood from the 10-year Fermi Large Area Telescope gamma-ray source catalog. Physical Review D. Published online April 20, 2021. doi: 10.1103/PhysRevD.103.083016.

M. Temming (2021, June 5). Antistars could lurk in Milky Way. Science News, 199(10), 8-9.

Space Travel Under Constant 1g Acceleration

The basic principle behind every high-thrust interplanetary space probe is to accelerate briefly and then coast, following an elliptical, parabolic, or mildly hyperbolic solar trajectory to your destination, using gravity assists whenever possible. But this is very slow.

Imagine, for a moment, that we have a spacecraft that is capable of a constant 1g (“one gee” = 9.8 m/s2) acceleration. Your spacecraft accelerates for the first half of the journey, and then decelerates for the second half of the journey to allow an extended visit at your destination. A constant 1g acceleration would afford human occupants the comfort of an earthlike gravitational environment where you would not be weightless except during very brief periods during the mission. Granted such a rocket ship would require a tremendous source of power, far beyond what today’s chemical rockets can deliver, but the day will come—perhaps even in our lifetimes—when probes and people will routinely travel the solar system in just a few days. Journeys to the stars, however, will be much more difficult.

The key to tomorrow’s space propulsion systems will be fusion and, later, matter-antimatter annihilation. The fusion of hydrogen into helium provides energy E = 0.008 mc2. This may not seem like much energy, but when today’s technological hurdles are overcome, fusion reactors will produce far more energy in a manner far safer than today’s fission reactors. Matter-antimatter annihilation, on the other hand, completely converts mass into energy in the amount given by Einstein’s famous equation E = mc2. You cannot get any more energy than this out of any conceivable on-board power or propulsion system. Of course, no system is perfect, so there will be some losses that will reduce the efficiency of even the best fusion or matter-antimatter propulsion system by a few percent.

How long would it take to travel from Earth to the Moon or any of the planets in our solar system under constant 1g acceleration for the first half of the journey and constant 1g deceleration during the second half of the journey? Using the equations below, you can calculate this easily.

Keep in mind that under a constant 1g acceleration, your velocity quickly becomes so great that you can assume a straight-line trajectory from point a to point b anywhere in our solar system.

Maximum velocity is reached at the halfway point (when you stop accelerating and begin decelerating) and is given by

The energy per unit mass needed for the trip (one way) is then given by

How much fuel will you need for the journey?

hydrogen fusion into helium gives: Efusion = 0.008 mfuel c2

matter-antimatter annihilation gives: Eanti = mfuel c2

This assumes 100% of the fuel goes into propelling the spacecraft, but of course there will be energy losses and operational energy requirements which will require a greater amount of fuel than this. Moreover, we are here calculating the amount of fuel you’ll need for each kg of payload. We would need to use calculus to determine how much additional energy will be needed to accelerate the ever changing amount of fuel as well. The journey may well be analogous to the traveler not being able to carry enough water to survive crossing the desert on foot.

Now, let’s use the equations above for a journey to the nearest stars. There are currently 58 known stars within 15 light years. The nearest is the triple star system Alpha Centauri A & B and Proxima Centauri (4.3 ly), and the farthest is LHS 292 (14.9 ly).

I predict that interstellar travel will remain impractical until we figure out a way to harness the vacuum energy of spacetime itself. If we could extract energy from the medium through which we travel, we wouldn’t need to carry fuel onboard the spacecraft.

We already do something analogous to this when we perform a gravity assist maneuver. As the illustration below shows, the spacecraft “borrows” energy by infinitesimally slowing down the much more massive Jupiter in its orbit around the Sun and transferring that energy to the tiny spacecraft so that it speeds up and changes direction. When the spacecraft leaves the gravitational sphere of influence of Jupiter, it is traveling just as fast as it did when it entered it, but now the spacecraft is farther from the Sun and moving faster than it would have otherwise.


Of course, our spacecraft will be “in the middle of nowhere” traveling through interstellar space, but what if space itself has energy we can borrow?