The white stripe is the imaginary "planetary belt" representing the zodiac. The star is of magnitude 1.0 and belongs to the constellation of Virgo. The red color symbolizes the vigor that is in the blood of the Paraense people.
The Republic was proclaimed in Pará on 16 November 1889. The following year, the municipalities of the state accepted the proposal of Governor Arturo Indio do Brazil to recognize the flag of the Republican Club as the official flag of the city of Belém. Afterwards the traditional republican symbol was adopted as the flag of the state. In it, the red symbolizes the revolutionary victory, valor, and blood, and the white band suggests a planetary belt representing the zodiac. The star symbolizes the constellation of Virgo.
According to information at www.brasilrepublica.hpg.ig.com.br, the flag was approved by the Pará state legislature on 3 June 1890 (but see below). It flew for the first time on the occasion of Pará's accession to the Republic of Brazil on 16 November 1889 as the symbol of the Paraense Republican Club. On 10 April 1890, the municipal council of Belém approved a proposal making the club's emblem the flag of the municipality of Belém. The decree that finally transformed the municipal flag into the state flag read as follows: The flag which served as the emblem of the Paraense Republican Club before the proclamation of the Republic, and which was adopted as the flag of the municipality at the session of 10 April 1890, is hereby to be considered the flag of the State of Pará.According to this site, the white stripe not only represents the belt of the Zodiac but also the equator and the Amazon River. The star, part of the constellation of Virgo, is also known as Spica, and symbolizes the position of Pará above the equatorial line, just as on the national flag Pará's star is the only one placed above the stripe inscribed Ordem e Progresso. The red field is for "the strength of Paraense blood, which runs through our veins as a true spirit of harmonized struggle, giving proof of the dedication of our patriots to the causes of Paraense support for Independence and for the Republic. The design of the flag is attributed to the republican Philadelfo Condurú.