12 ASTRONOMY • JANUARY 2022
14 Leo I
As the name implies, Leo I resides in the zodiacal constella-
tion Leo the Lion. This deep-sky target is a great example of a
dwarf spheroidal galaxy. Such objects are faint, have little dust,
and are not forming new stars. Usually, they are found as satel-
lites orbiting larger galaxies — more than 30 of the 59 confirmed
satellites of the Milky Way are dwarf spheroidal galaxies.
Leo I lies approximately 820,000 light-years away, which
makes it the fourth most distant satellite of our galaxy. It has
a diameter on the order of 2,000 light-years and a mass of13 The Crescent Nebula
The Crescent Nebula, which lies in the constellation Cygnus the Swan, has plenty of names.
Among them are NGC 6888, Caldwell 27, Sharpless 2–105, and LBN 203. Whatever you call it,
this object is a fascinating bubble of gas being carved out of the interstellar medium by the
winds of an intensely hot star named WR 136.
As this high-velocity wind hits a slower-moving stellar wind that the star previously pro-
duced when it became a red giant, the collision produces a shock front and the shell of gas
we see: NGC 6888. The interaction is so energetic, in fact, that it also produces X-rays.
The Crescent lies some 5,000 light-years away and measures about 25 light-years across.
It glows at magnitude 7.4. That seems like a healthy number (just below naked-eye visibility);
however, the nebula’s size (0.3° by 0.2°) really cuts down the surface brightness. You’ll begin
to see some of the structure through an 8-inch telescope. But, honestly, it takes an 11-inch or
larger instrument to begin to do it justice.
To find NGC 6888, point your telescope 1.2° west-northwest of the magnitude 4.8 star
34 Cygni. Or, if you want to start with a brighter star, aim 2.7° southwest of magnitude 2.2 Sadr
(Gamma [γ] Cygni).
The slightly curved northwest edge is the brightest, but a short line of bright nebulosity also
sits at the southwest edge. Larger telescopes will show a thick nebulous patch that runs from
the westernmost edge to the central star. You’ll see WR 136 easily. It shines at 7th magnitude
and lies at NGC 6888’s center.
An Oxygen-III filter really helps to bring out the contrasting sections of this object. This filter
also increases its overall visibility by dimming the vast number of background stars that are
part of the Milky Way running through Cygnus. — M.B.12 3C 273
Observers love looking at distant objects —
and the farther, the better. Well, how about an
object that’s more than 2 billion light-years
away and visible through an 8-inch telescope?
3C 273 is that object. It’s a quasar: a superlumi-
nous active galactic nucleus.
At the center of the celestial wonder sits
a supermassive black hole that weighs in at
about 1 billion solar masses. This behemoth has
lots of gas around it. As the gravity from the
black hole pulls the gas in, it forms a swirling
disk. A huge amount of energy is then gener-
ated by friction and other forces in the disk.
Quasars are the most luminous objects
in the universe, and thus visible across vast
distances. The brightest known quasar, 3C 273
also has the distinction of being the first quasar
ever identified.
3C 273 lies in the constellation Virgo the
Maiden. It appears starlike and glows softly at
magnitude 12.9. A good star chart, however,
will allow you to quickly identify it. Begin
your search 4.7° northwest of magnitude 2.
Gamma (γ) Virginis.
Astronomers can’t determine the distance
to 3C 273 directly, so they infer it by measur-
ing its redshift, which is the amount its light is
stretched due to its motion away from us. The
greater the redshift, the more distant the object
is. 3C 273 has a redshift of 0.158, which corre-
sponds to a distance of 2.443 billion light-years.
Let’s close with a number that properly dem-
onstrates the stunning brightness of 3C 273:
The object is 3.99 trillion times as bright as our
daytime star. — M.B.STEVE WALTERSJOHN CHUMACK