Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A review of the W51 Cloud   The W51 cloud complex is one of the best laboratories in our Galaxy to study
high-mass star formation. At a distance of about 5 kpc, it is the closest
region containing a high-mass protocluster, and it has two. The cloud includes
a long infrared-dark cloud, is interacting with a supernova remnant, and
contains a variety of unique massive protostellar sources. This article is an
observational review of the region.

23
The categories of this paper are: solv-int
34
The categories of this paper are: chem-ph
11
The categories of this paper are: hep-lat
8
The categories of this paper are: hep-ex
22
The categories of this paper are: chao-dyn
37
The categories of this paper are: supr-con
17
The categories of this paper are: q-fin
10
The categories of this paper are: q-bio
18
The categories of this paper are: econ
25
The categories of this paper are: patt-sol
15
The categories of this paper are: nucl-ex
26
The categories of this paper are: bayes-an
33
The categories of this paper are: plasm-ph
16
The categories of this paper are: stat
0
The categories of this paper are: hep-ph
28
The categories of this paper are: alg-geom