Sky & Telescope - July 2024 - 7

matter and gravity are dispersed, and
matter density is insufficient to fight
the negative or vacuum energy we now
call dark energy?
Ven Challa, MD
Winston-Salem, North Carolina
"
Camille M. Carlisle replies: You are
asking questions that people have
asked in one form or another for millennia
- which is to say that they are fundamental
to our growing understanding of the
cosmos. Augustine of Hippo, for example,
pondered the nature of time in his 4thcentury
autobiography The Confessions
of Saint Augustine (see Book XI), and his
thoughts wouldn't have been totally foreign
to modern cosmologists.
To do your questions justice would
take several books, and so in lieu of a
short, unsatisfying response, I direct you
to other more prolific authors. I suggest
starting with Brian Greene's The Hidden
Reality, which discusses the questions
you pose in the framework of string theory
and multiple universes. (We don't know
whether that framework is correct, but it's a
useful framework for this discussion.) Matt
Strassler's new book Waves in an Impossible
Sea (S&T: June 2024, p. 61), which
explores cosmological questions from the
particle-physics side of things, also might
be of interest.
Magnetic Magnetars
Zach Cano's article " Adventures
of a Millisecond Magnetar " (S&T:
Apr. 2024, p. 34) really helped me
to understand the possible " lives " of
neutron stars, millisecond magnetars,
long gamma-ray bursts, and fast radio
bursts. I've read articles that refer to
them and sort of explain what they are,
but his article was so helpful to put it
all together.
Steve Carnes
North Oaks, Minnesota
"
Zach Cano replies: Thank you for
your comments on the recent article
in Sky & Telescope about the possible ways
that neutron stars continue to interact with
their environments and other stellar objects
long after their progenitor star has suffered
its final demise. The article provides
the merest of glimpses into a great body
of research being undertaken by many talented
and knowledgeable astronomers and
astrophysicists around the world.
I am grateful for the chance to write
about these intriguing objects in a way
that provides a colorful summary of what
we know about them, and I am even more
grateful I have managed to share this interest
with the public.
FOR THE RECORD
* The Crab Nebula is roughly 600 years
older than Cassiopeia A, not 1,500 years
older (S&T: May 2024, p. 19).
SUBMISSIONS: Write to Sky & Telescope, 1374 Massachusetts Ave., 4th Floor, Cambridge, MA 02138, USA, or email:
letters@skyandtelescope.org. Please limit your comments to 250 words; letters may be edited for brevity and clarity.
75, 50 & 25 YEARS AGO by Roger W. Sinnott
The advent of atomic clocks led
1949
1974
º July 1949
Keeping Time " For many years,
at the United States Naval Observatory
and other national observatories
where time is recorded
and dispensed, quartz-crystalcontrolled
clocks have been used
to keep time as accurately as does
the earth itself - to about 1/1,000
of a second per day. Nevertheless,
quartz-crystal clocks are subject
to vagaries that have indicated the
need for even more accurate means
of control. This has now been found
in what is known as the 'atomic
clock,' developed at the National
Bureau of Standards by Dr. Harold
1999
Lyons [and colleagues].
" The frequency of an absorption
line [of] ammonia . . . is now used as
the 'governor' on apparatus capable
of time constancy of one part in 10
million . . . In addition to its obvious
astronomical applications, such as
furnishing an invariant check of the
earth's rotation, the atomic clock
will [aid] navigation systems. "
to the occasional insertion of leap
seconds in the 24-hour day.
º July 1974
Earth Grazer " The great fireball
observed on August 10, 1972, which
streaked across the early afternoon
skies of the western mountain
states from Utah to Montana and
over . . . Alberta, was a unique
object [see cover shot at left]. It
entered the atmosphere at such a
small angle with the earth's surface
that it did not quite make it to the
ground. Instead, after dazzling
thousands of astonished spectators
for well over a minute, it left the
atmosphere to continue its travel
around the sun . . . I was one of the
astonished spectators . . .
" If it was of magnitude -15 as
seen from a distance of 100 kilometers,
the mass was 4,000 tons and
the diameter 13 meters. "
In a stunning coincidence,
Harvard-Smithsonian astronomer
Luigi Jacchia, a meteor expert, was
vacationing that day at Jackson
Lake Lodge, Wyoming. He gave our
readers his analysis.
º July 1999
Hypernovae " What packs more
punch than a supernova? Something
does, though no one knows
for sure exactly what. Northwestern
University astronomer Q. Daniel
Wang claims to have found the
first clear evidence for 'hypernovae':
stellar explosions carrying as
much as 100 times the energy of
a supernova or even more. That's
up in the same energy range as
cosmic gamma-ray bursts, whose
power has awed and confounded
astronomers in recent years . . .
Many now suspect that hypernovae
and gamma-ray bursts are one
and the same.
" Wang's evidence consists of
two expanding gas bubbles that he
discovered in extremely deep X-ray
images of the galaxy M101 taken
with the Rosat satellite. "
Starting in 2003, some " long "
gamma-ray bursts have been definitely
connected with hypernovae.
sk yand tele scope .o r g * JULY 2024
7

http://skyandtelescope.org

Sky & Telescope - July 2024

Table of Contents for the Digital Edition of Sky & Telescope - July 2024