Exhibit 5.1

                            Opinion of Ropes & Gray

                                                                 August 17, 1999
Securities and Exchange Commission
450 Fifth Street, N.W.
Washington, DC  20549

  Re:  NSTAR Registration on Form S-8

Ladies and Gentlemen:

  We have acted as counsel to NSTAR (the "Registrant"), a Massachusetts business
trust, in connection with the Registrant's Registration Statement on Form S-8.
The Registration Statement is to be filed with the Securities and Exchange
Commission and relates to the registration under the Securities Act of 1933, as
amended, of 525,000 Common Shares, par value $1.00 per share (the "Shares"), in
connection with the Employee Savings Plan of Commonwealth Energy System and
Subsidiary Companies (collectively, the "Plan").

  We are familiar with the Registration Statement and the Plan, and we have
examined the Registrant's Declaration of Trust, dated April 20, 1999, and the
Registrant's By-Laws.  We have also examined such other documents, instruments,
certificates and corporate records as we considered necessary for purposes of
this opinion, and have assumed that the signatures on all documents examined by
us are genuine.

  Based on the foregoing, we are of the opinion that the Shares, when issued in
accordance with the Plan and the consideration therefore received by the
Registrant will be authorized, validly issued, fully paid and nonassessable.

  We hereby consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement.

                                            Very truly yours,

                                            /s/ Ropes & Gray

                                            Ropes & Gray