Sky and Telescope - May 2015 - 24

Photographic Plate Science
computer to run it, and hard drives to store the data.
The project is more than simply making digital copies
of the plates. The software processing provides photometric data for all resolved stars, and astronomers can
access the resulting light curves from the DASCH project webpage by entering either an object name or right
ascension and declination coordinates. The digitized
images of the plates are also available for download.
The project is on track to ﬁnish digitizing the plates
in 2017, an eﬀort spanning more than ten years.

Unsolved Stellar Cases

Mary Alexandra Agner writes science nonﬁction, ﬁction,
and poetry in Somerville, Massachusetts.

DASCH

Although some of the digital data are already online,
Schaefer prefers to use the plates the way Henrietta
Leavitt and associates did: in person, in the collection,
putting the plates on light tables, and estimating the
photometry by eye. And while the Stingray Nebula, a
planetary nebula in the constellation Ara, has a lot of
eye appeal, it was the behavior of its progenitor star that
brought Schaefer to the plates for another of his projects.
In 1967, the Stingray Nebula was a star, Henize 3-1357,
identiﬁed by its spectrum as a B-type supergiant. In
1989, the spectrum changed, showing planetary nebula
emission lines and indicating Hen 3-1357 had thrown oﬀ
its outer layers and was ionizing them, resulting in the
boxy shape later imaged by the Hubble Space Telescope.
"The Stingray Nebula is the only case of humanity
seeing a planetary nebula birth," Schaefer says. Because
of the Harvard plates, astronomers have a photometric

history that tracks the star's conversion to a planetary
nebula, for use in tandem with spectral data.
Schaefer's results from the plate collection are bafﬂing: Hen 3-1357 dimmed by nearly one magnitude
from 1885 to 1992. Furthermore, the rate of dimming
increased in the late 1970s, speeding up to approximately
0.2 magnitude per year.
Although existing theory can explain Hen 3-1357's
fade after its emission lines appeared, the dimming it
exhibited before that is a mystery: the star's luminosity
dropped by an unprecedented 40% in that time period,
corresponding to a roughly 20% shrink in radius.
Depending on a star's mass and how fast the star loses
its outer layers, it can take a few years to a few decades
to transition to a planetary nebula. But astronomers
don't understand the details, and it's unclear what Hen
3-1357's prolonged fading means. One possibility Schaefer raises is that this object somehow "anticipated" it was
about to become a planetary nebula and started dimming in advance, a transition recorded in the plates.
Although digitization might soon make photographic
plate technology obsolete, the detailed information on
the plates won't go stale. Their timespan, while short
compared to some astronomical events, enables investigations into some of the most interesting and explosive
astronomical phenomena. ✦

Call for Volunteers: DASCH Project
We are looking for help transcribing observing logbooks and other historic astronomical documents as part of the Digital Access
to a Sky Century at Harvard (DASCH)
project. The project aims to digitize and
make available online fully processed digital
images and data from the Harvard College
Observatory's roughly 500,000 plates.
The Harvard-Smithsonian Center for
Astrophysics (CfA), the Astrophysics Data
System (ADS), the CfA's John G. Wolbach
Library, and the Smithsonian Transcription
Center have banded together to recruit
volunteers to help transcribe the associated
logbooks. Thanks to help from volunteers,
staﬀ, and others, we've already transcribed
700 of the 800 books, which have enabled

May 2015 sky & telescope

the digitization of 95,000 glass plates. Using
both the transcribed logbook entries and the
plates, the DASCH team has already discovered new types of variable stars and novae,
explored the long-term stability of exoplanet
host stars, and identiﬁed historic outbursts
from black hole X-ray binaries.
We still need help transcribing the last
100 of the 800 logbooks. We have posted
high-resolution scans of all the pages of
these ﬁnal logbooks at http://transcription.
si.edu/ (look for the Harvard-Smithsonian
Center for Astrophysics project). We will
also add materials from other HCO records,
which include details of the ﬁrst observations of Saturn's moon Hyperion in 1848,
the discovery of the Horsehead Nebula in

1888, and correspondence between the
women working as "computers" during the
Edward Pickering era at the observatory.
We ask amateur and professional astronomers from around the world to visit the site
and help expedite the transcription process.
Scientiﬁcally speaking, this archive is the
only repository of astronomical data of its
kind, and digitizing it will enable astronomers to probe longstanding astrophysical
puzzles and make exciting new discoveries.
Please lend a hand and help us complete
this important work!
Telescope engineer David Sliski (University of
Pennsylvania) initiated DASCH's transcription
project. Jonathan Grindlay directs DASCH.

http://transcription.si.edu/ http://transcription.si.edu/

Sky and Telescope - May 2015

Table of Contents for the Digital Edition of Sky and Telescope - May 2015